diff --git a/TODO.md b/TODO.md
index e8dd4f6..1a1b960 100644
--- a/TODO.md
+++ b/TODO.md
@@ -22,6 +22,16 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made `s3.part_size` set the multipart upload part size
+  ([issue #232](https://git.eeqj.de/sneak/vaultik/issues/232)). It was
+  loaded and defaulted but never passed to the S3 client, whose uploader
+  used a fixed 10MiB part. It now reaches the uploader for `storage_url`
+  and for the `s3.*` fields, and a part size S3 refuses, below 5MiB or
+  above 5GiB, `0` included, fails at config load. A blob too large for
+  S3's limit of 10,000 parts at the configured size is uploaded in larger
+  parts. The docs gave the default as `5MB`, which the config file reads
+  as 5,000,000 bytes, below the minimum; they now say `5MiB`.
+
 - 2026-10-07: Made per-name retention work when the hostname contains `_`
   ([issue #230](https://git.eeqj.de/sneak/vaultik/issues/230)). A
   snapshot ID is `hostname_name_timestamp`, and the name was read as
diff --git a/config.example.yml b/config.example.yml
index 6dc50de..3ad5870 100644
--- a/config.example.yml
+++ b/config.example.yml
@@ -287,10 +287,11 @@ storage_url: "rclone://myremote/path/to/backups"
 #  #use_ssl: true
 #
 #  # Part size for multipart uploads
-#  # Minimum 5MB, affects memory usage during upload
-#  # Supports: 5MB, 10M, 100MiB, etc.
-#  # Default: 5MB
-#  #part_size: 5MB
+#  # Minimum 5MiB, maximum 5GiB; affects memory usage during upload
+#  # A blob too large for 10,000 parts of this size gets larger parts
+#  # Supports: 10MB, 16MiB, 100MiB, etc. (5MB is below the minimum)
+#  # Default: 5MiB
+#  #part_size: 5MiB
 
 # Path to local SQLite index database
 # This database tracks file state for incremental backups
diff --git a/internal/cli/config.go b/internal/cli/config.go
index b00a602..545f81a 100644
--- a/internal/cli/config.go
+++ b/internal/cli/config.go
@@ -205,7 +205,7 @@ storage_url: ""
 #   access_key_id: YOUR_ACCESS_KEY
 #   secret_access_key: YOUR_SECRET_KEY
 #   # region: us-east-1        # Default: us-east-1
-#   # part_size: 5MB           # Multipart upload part size. Default: 5MB
+#   # part_size: 5MiB          # Upload part size, 5MiB to 5GiB. Default: 5MiB
 #   # For the s3:// form, disable TLS with ?ssl=false in the URL, not use_ssl.
 
 # ─── OPTIONAL ────────────────────────────────────────────────────────────────
diff --git a/internal/config/config.go b/internal/config/config.go
index 1448a32..e5192b0 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -33,11 +33,14 @@ const secretKeyPrefix = "AGE-SECRET-KEY-"
 const (
 	defaultBlobSizeLimit    = Size(10 * 1024 * 1024 * 1024) // 10GB
 	defaultChunkSize        = Size(10 * 1024 * 1024)        // 10MB
-	defaultS3PartSize       = Size(5 * 1024 * 1024)         // 5MB
+	defaultS3PartSize       = Size(5 * 1024 * 1024)         // 5MiB
 	defaultCompressionLevel = 3
 	minChunkSize            = 1024 * 1024 // 1MB
 	minCompressionLevel     = 1
 	maxCompressionLevel     = 19
+	// S3 accepts a multipart upload part from 5MiB to 5GiB.
+	minS3PartSize = 5 * 1024 * 1024
+	maxS3PartSize = 5 * 1024 * 1024 * 1024
 )
 
 // Sentinel validation errors.
@@ -55,6 +58,7 @@ var (
 		"blob_size_limit must be at least the largest chunk the chunker can " +
 			"emit (chunk_size times the FastCDC size spread)")
 	errBadCompression   = errors.New("compression_level must be between 1 and 19")
+	errBadS3PartSize    = errors.New("s3.part_size must be between 5MiB and 5GiB")
 	errBadStorageScheme = errors.New(
 		"storage_url must start with s3://, file://, or rclone://")
 	errStorageNotConfigured = errors.New(
@@ -244,6 +248,7 @@ func Load(path string) (*Config, error) {
 		ChunkSize:        defaultChunkSize,
 		IndexPath:        filepath.Join(xdg.DataHome, appName, "index.sqlite"),
 		CompressionLevel: defaultCompressionLevel,
+		S3:               S3Config{PartSize: defaultS3PartSize},
 	}
 
 	// Convert smartconfig data to YAML then unmarshal
@@ -294,10 +299,6 @@ func Load(path string) (*Config, error) {
 		cfg.S3.Region = "us-east-1"
 	}
 
-	if cfg.S3.PartSize == 0 {
-		cfg.S3.PartSize = defaultS3PartSize
-	}
-
 	// Check config file permissions (warn if world or group readable)
 	//nolint:gosec // G703: config path is operator-supplied by design
 	info, statErr := os.Stat(path)
@@ -332,6 +333,7 @@ func Load(path string) (*Config, error) {
 //     (chunk_size times chunker.ChunkSizeSpread), so a single-chunk blob never
 //     exceeds the configured limit
 //   - Compression level must be between 1 and 19
+//   - S3 part size must be between 5MiB and 5GiB, the part sizes S3 accepts
 //
 // Returns an error describing the first validation failure encountered.
 func (c *Config) Validate() error {
@@ -376,6 +378,11 @@ func (c *Config) Validate() error {
 		return errBadCompression
 	}
 
+	if c.S3.PartSize.Int64() < minS3PartSize ||
+		c.S3.PartSize.Int64() > maxS3PartSize {
+		return errBadS3PartSize
+	}
+
 	return nil
 }
 
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 52b46ad..c9bb10d 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -166,6 +166,7 @@ func TestValidateBlobSizeLimit(t *testing.T) {
 			ChunkSize:        chunkSize,
 			BlobSizeLimit:    blobLimit,
 			CompressionLevel: 3,
+			S3:               S3Config{PartSize: defaultS3PartSize},
 		}
 	}
 
@@ -221,6 +222,120 @@ func TestValidateBlobSizeLimit(t *testing.T) {
 	}
 }
 
+// TestValidateS3PartSize checks that s3.part_size is held to the part sizes
+// S3 accepts, 5MiB to 5GiB, by changing only the part size of the test
+// config. "5MB" in the config file is 5,000,000 bytes, below the minimum.
+func TestValidateS3PartSize(t *testing.T) {
+	t.Parallel()
+
+	base, err := Load(os.Getenv("VAULTIK_CONFIG"))
+	if err != nil {
+		t.Fatalf("Failed to load config: %v", err)
+	}
+
+	tests := []struct {
+		name     string
+		partSize Size
+		wantErr  bool
+	}{
+		{
+			name:     "5MB is rejected",
+			partSize: 5_000_000,
+			wantErr:  true,
+		},
+		{
+			name:     "one byte below 5MiB is rejected",
+			partSize: minS3PartSize - 1,
+			wantErr:  true,
+		},
+		{
+			name:     "5MiB is accepted",
+			partSize: minS3PartSize,
+			wantErr:  false,
+		},
+		{
+			name:     "5GiB is accepted",
+			partSize: maxS3PartSize,
+			wantErr:  false,
+		},
+		{
+			name:     "one byte above 5GiB is rejected",
+			partSize: maxS3PartSize + 1,
+			wantErr:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			cfg := *base
+			cfg.S3.PartSize = tt.partSize
+
+			err := cfg.Validate()
+			if tt.wantErr {
+				if !errors.Is(err, errBadS3PartSize) {
+					t.Fatalf("Validate() error = %v, want errBadS3PartSize", err)
+				}
+
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Validate() unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+// TestLoadS3PartSize checks that a config file without s3.part_size loads
+// with the 5MiB default, and that an explicit 0 fails at load like any other
+// part size S3 refuses.
+func TestLoadS3PartSize(t *testing.T) {
+	t.Parallel()
+
+	const withoutPartSize = "snapshots:\n" +
+		"  test:\n" +
+		"    paths: [/tmp/vaultik-test-source]\n" +
+		"storage_url: file:///tmp/vaultik-test-storage\n"
+
+	writeConfig := func(t *testing.T, text string) string {
+		t.Helper()
+
+		path := filepath.Join(t.TempDir(), "config.yml")
+
+		err := os.WriteFile(path, []byte(text), 0o600)
+		if err != nil {
+			t.Fatalf("write config: %v", err)
+		}
+
+		return path
+	}
+
+	t.Run("absent loads as 5MiB", func(t *testing.T) {
+		t.Parallel()
+
+		cfg, err := Load(writeConfig(t, withoutPartSize))
+		if err != nil {
+			t.Fatalf("Load() unexpected error: %v", err)
+		}
+
+		if cfg.S3.PartSize != defaultS3PartSize {
+			t.Errorf("s3.part_size = %d, want %d",
+				cfg.S3.PartSize, defaultS3PartSize)
+		}
+	})
+
+	t.Run("0 is rejected", func(t *testing.T) {
+		t.Parallel()
+
+		_, err := Load(writeConfig(t, withoutPartSize+"s3:\n  part_size: 0\n"))
+		if !errors.Is(err, errBadS3PartSize) {
+			t.Fatalf("Load() error = %v, want errBadS3PartSize", err)
+		}
+	})
+}
+
 // TestValidateAgeRecipients checks that recipients are parsed at config load
 // (a bad entry fails immediately, not mid-backup) and that no invalid entry —
 // least of all a pasted secret key — is echoed in the error. An empty list
@@ -236,6 +351,7 @@ func TestValidateAgeRecipients(t *testing.T) {
 			ChunkSize:        Size(10 * 1024 * 1024),
 			BlobSizeLimit:    Size(10 * 1024 * 1024 * 1024),
 			CompressionLevel: 3,
+			S3:               S3Config{PartSize: defaultS3PartSize},
 		}
 	}
 
diff --git a/internal/s3/client.go b/internal/s3/client.go
index 43f8725..f5fc91c 100644
--- a/internal/s3/client.go
+++ b/internal/s3/client.go
@@ -27,10 +27,12 @@ type Client struct {
 	bucket   string
 	prefix   string
 	endpoint string
+	partSize int64
 }
 
 // Config contains S3 client configuration.
-// All fields are required except Prefix, which defaults to an empty string.
+// All fields are required except Prefix, which defaults to an empty string,
+// and PartSize, where zero means the SDK default of 5 MiB.
 // A non-empty Prefix is joined to every key with one "/", whether or not
 // it ends with one.
 // The Endpoint field should include the protocol (http:// or https://).
@@ -41,6 +43,9 @@ type Config struct {
 	AccessKeyID     string
 	SecretAccessKey string
 	Region          string
+	// PartSize is the size in bytes of each part of a multipart upload.
+	// An upload too large for S3's limit of 10,000 parts gets larger parts.
+	PartSize int64
 }
 
 // nopLogger is a logger that discards all output.
@@ -90,6 +95,7 @@ func NewClient(ctx context.Context, cfg Config) (*Client, error) {
 		bucket:   cfg.Bucket,
 		prefix:   prefix,
 		endpoint: cfg.Endpoint,
+		partSize: cfg.PartSize,
 	}, nil
 }
 
@@ -123,12 +129,9 @@ func (c *Client) PutObjectWithProgress(
 ) error {
 	fullKey := c.prefix + key
 
-	// uploadPartSize is 10MB for better progress granularity.
-	const uploadPartSize = 10 * 1024 * 1024
-
 	// Create an uploader with the S3 client
 	uploader := manager.NewUploader(c.s3Client, func(u *manager.Uploader) {
-		u.PartSize = uploadPartSize
+		u.PartSize = uploadPartSize(c.partSize, size)
 	})
 
 	// Create a progress reader that tracks upload progress
@@ -149,6 +152,21 @@ func (c *Client) PutObjectWithProgress(
 	return err
 }
 
+// uploadPartSize returns the part size for an upload of size bytes: the
+// configured part size (the SDK default when zero), raised where needed so
+// the upload fits in S3's limit of 10,000 parts. The uploader cannot raise
+// it itself, because it cannot seek the progress reader to learn its size.
+func uploadPartSize(configured, size int64) int64 {
+	if configured == 0 {
+		configured = manager.DefaultUploadPartSize
+	}
+
+	maxParts := int64(manager.MaxUploadParts)
+	smallestThatFits := (size + maxParts - 1) / maxParts // rounded up
+
+	return max(configured, smallestThatFits)
+}
+
 // GetObject downloads an object from S3 with the specified key.
 // The key is automatically prefixed with the configured prefix.
 // Returns a ReadCloser containing the object data. The caller must
diff --git a/internal/s3/client_internal_test.go b/internal/s3/client_internal_test.go
new file mode 100644
index 0000000..552059d
--- /dev/null
+++ b/internal/s3/client_internal_test.go
@@ -0,0 +1,55 @@
+package s3
+
+import "testing"
+
+// TestUploadPartSize checks that an upload too large for 10,000 parts of the
+// configured size gets parts just large enough to fit in 10,000.
+func TestUploadPartSize(t *testing.T) {
+	t.Parallel()
+
+	const mib = 1024 * 1024
+
+	tests := []struct {
+		name       string
+		configured int64
+		size       int64
+		want       int64
+	}{
+		{
+			name:       "an upload that fits keeps the configured size",
+			configured: 5 * mib,
+			size:       10 * 1024 * mib,
+			want:       5 * mib,
+		},
+		{
+			name:       "exactly 10,000 parts keeps the configured size",
+			configured: 6 * mib,
+			size:       10_000 * 6 * mib,
+			want:       6 * mib,
+		},
+		{
+			name:       "one byte more than 10,000 parts adds a byte to each",
+			configured: 6 * mib,
+			size:       10_000*6*mib + 1,
+			want:       6*mib + 1,
+		},
+		{
+			name:       "zero means the SDK default of 5MiB",
+			configured: 0,
+			size:       1,
+			want:       5 * mib,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := uploadPartSize(tt.configured, tt.size)
+			if got != tt.want {
+				t.Errorf("uploadPartSize(%d, %d) = %d, want %d",
+					tt.configured, tt.size, got, tt.want)
+			}
+		})
+	}
+}
diff --git a/internal/s3/module.go b/internal/s3/module.go
index 33b24ba..71f1b88 100644
--- a/internal/s3/module.go
+++ b/internal/s3/module.go
@@ -28,6 +28,7 @@ func provideClient(lc fx.Lifecycle, cfg *config.Config) (*Client, error) {
 		AccessKeyID:     cfg.S3.AccessKeyID,
 		SecretAccessKey: cfg.S3.SecretAccessKey,
 		Region:          cfg.S3.Region,
+		PartSize:        cfg.S3.PartSize.Int64(),
 	})
 	if err != nil {
 		return nil, err
diff --git a/internal/storage/module.go b/internal/storage/module.go
index 0e400fd..8d00ea1 100644
--- a/internal/storage/module.go
+++ b/internal/storage/module.go
@@ -99,6 +99,7 @@ func storerFromParsedS3URL(parsed *URL, cfg *config.Config) (Storer, error) {
 		AccessKeyID:     cfg.S3.AccessKeyID,
 		SecretAccessKey: cfg.S3.SecretAccessKey,
 		Region:          region,
+		PartSize:        cfg.S3.PartSize.Int64(),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("creating S3 client: %w", err)
@@ -134,6 +135,7 @@ func storerFromLegacyS3Config(cfg *config.Config) (Storer, error) {
 		AccessKeyID:     cfg.S3.AccessKeyID,
 		SecretAccessKey: cfg.S3.SecretAccessKey,
 		Region:          region,
+		PartSize:        cfg.S3.PartSize.Int64(),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("creating S3 client: %w", err)
diff --git a/internal/storage/s3_test.go b/internal/storage/s3_test.go
index 2d581f7..2434eb5 100644
--- a/internal/storage/s3_test.go
+++ b/internal/storage/s3_test.go
@@ -1,11 +1,14 @@
 package storage_test
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"net/http"
 	"net/http/httptest"
 	"slices"
 	"strings"
+	"sync/atomic"
 	"testing"
 
 	"github.com/johannesboyne/gofakes3"
@@ -19,6 +22,13 @@ import (
 // s3TestBucket is the bucket created for each in-process S3 server.
 const s3TestBucket = "test-bucket"
 
+// Credentials for the tests that build a storer from a config.Config. The
+// in-process S3 server accepts any.
+const (
+	s3TestAccessKeyID     = "key"
+	s3TestSecretAccessKey = "secret"
+)
+
 // newS3Storer builds an s3:// backend backed by a fresh in-process
 // S3 server. It reuses the same in-memory S3 harness (gofakes3 + s3mem
 // over httptest) that internal/s3 and the not-found regression test use,
@@ -128,8 +138,8 @@ func TestS3URLPrefixKeyLayout(t *testing.T) {
 			storer, err := storage.NewStorer(&config.Config{
 				StorageURL: storageURL + "?endpoint=" + srv.URL,
 				S3: config.S3Config{
-					AccessKeyID:     "key",
-					SecretAccessKey: "secret",
+					AccessKeyID:     s3TestAccessKeyID,
+					SecretAccessKey: s3TestSecretAccessKey,
 				},
 			})
 			if err != nil {
@@ -171,6 +181,90 @@ func TestS3URLPrefixKeyLayout(t *testing.T) {
 	}
 }
 
+// TestS3UploadUsesConfiguredPartSize checks that s3.part_size reaches the
+// multipart uploader, through storage_url and through the s3.* fields. An
+// object three parts long must arrive as three parts; at the SDK's default
+// of 5 MiB it would arrive as four.
+func TestS3UploadUsesConfiguredPartSize(t *testing.T) {
+	t.Parallel()
+
+	const (
+		partSize  = 6 * 1024 * 1024
+		wantParts = 3
+	)
+
+	backend := s3mem.New()
+
+	err := backend.CreateBucket(s3TestBucket)
+	if err != nil {
+		t.Fatalf("create bucket: %v", err)
+	}
+
+	// Every part of a multipart upload is one request with a partNumber.
+	var parts atomic.Int32
+
+	fake := gofakes3.New(backend).Server()
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Query().Has("partNumber") {
+				parts.Add(1)
+			}
+
+			fake.ServeHTTP(w, r)
+		}))
+	t.Cleanup(srv.Close)
+
+	cases := []struct {
+		name string
+		cfg  *config.Config
+	}{
+		{
+			name: "storage_url",
+			cfg: &config.Config{
+				StorageURL: "s3://" + s3TestBucket + "?endpoint=" + srv.URL,
+				S3: config.S3Config{
+					AccessKeyID:     s3TestAccessKeyID,
+					SecretAccessKey: s3TestSecretAccessKey,
+					PartSize:        partSize,
+				},
+			},
+		},
+		{
+			name: "s3.endpoint",
+			cfg: &config.Config{
+				S3: config.S3Config{
+					Endpoint:        srv.URL,
+					Bucket:          s3TestBucket,
+					AccessKeyID:     s3TestAccessKeyID,
+					SecretAccessKey: s3TestSecretAccessKey,
+					PartSize:        partSize,
+				},
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		parts.Store(0)
+
+		storer, err := storage.NewStorer(tc.cfg)
+		if err != nil {
+			t.Fatalf("%s: NewStorer: %v", tc.name, err)
+		}
+
+		data := bytes.NewReader(make([]byte, wantParts*partSize))
+
+		err = storer.PutWithProgress(
+			context.Background(), "blob", data, data.Size(), nil)
+		if err != nil {
+			t.Fatalf("%s: PutWithProgress: %v", tc.name, err)
+		}
+
+		if got := parts.Load(); got != wantParts {
+			t.Errorf("%s: uploaded in %d parts, want %d", tc.name, got, wantParts)
+		}
+	}
+}
+
 // listStreamKeys returns the keys ListStream yields under a prefix, and
 // fails the test on a listing error.
 func listStreamKeys(t *testing.T, s storage.Storer, prefix string) []string {
diff --git a/test/config.yaml b/test/config.yaml
index f67577f..cd0e303 100644
--- a/test/config.yaml
+++ b/test/config.yaml
@@ -19,7 +19,7 @@ s3:
   secret_access_key: test-secret-key
   region: us-east-1
   use_ssl: true
-  part_size: 5242880  # 5MB
+  part_size: 5242880  # 5MiB
 index_path: /tmp/vaultik-test.sqlite
 chunk_size: 10MB
 blob_size_limit: 10GB