diff --git a/pkg/sif/testdata/TestAppendImage/Default.golden b/pkg/sif/testdata/TestAppendImage/Default.golden
new file mode 100644
index 0000000..2d28e07
Binary files /dev/null and b/pkg/sif/testdata/TestAppendImage/Default.golden differ
diff --git a/pkg/sif/testdata/TestAppendImage/WithReference.golden b/pkg/sif/testdata/TestAppendImage/WithReference.golden
new file mode 100644
index 0000000..a056e1c
Binary files /dev/null and b/pkg/sif/testdata/TestAppendImage/WithReference.golden differ
diff --git a/pkg/sif/testdata/TestAppendIndex/Default.golden b/pkg/sif/testdata/TestAppendIndex/Default.golden
new file mode 100644
index 0000000..d188796
Binary files /dev/null and b/pkg/sif/testdata/TestAppendIndex/Default.golden differ
diff --git a/pkg/sif/testdata/TestAppendIndex/WithReference.golden b/pkg/sif/testdata/TestAppendIndex/WithReference.golden
new file mode 100644
index 0000000..b32f10e
Binary files /dev/null and b/pkg/sif/testdata/TestAppendIndex/WithReference.golden differ
diff --git a/pkg/sif/update.go b/pkg/sif/update.go
index 39b0d58..8bd5e0f 100644
--- a/pkg/sif/update.go
+++ b/pkg/sif/update.go
@@ -11,7 +11,9 @@ import (
 	"path/filepath"
 	"slices"
 
+	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/mutate"
 	"github.com/google/go-containerregistry/pkg/v1/types"
 	"github.com/sylabs/sif/v2/pkg/sif"
 )
@@ -330,3 +332,69 @@ func selectBlobsExcept(keep []v1.Hash) sif.DescriptorSelectorFunc {
 		return false, nil
 	}
 }
+
+// appendOpts accumulates append options.
+type appendOpts struct {
+	tempDir string
+	ref     name.Reference
+}
+
+// AppendOpt are used to specify options to apply when appending to a SIF.
+type AppendOpt func(*appendOpts) error
+
+// OptAppendTempDir sets the directory to use for temporary files. If not set, the
+// directory returned by os.TempDir is used.
+func OptAppendTempDir(d string) AppendOpt {
+	return func(c *appendOpts) error {
+		c.tempDir = d
+		return nil
+	}
+}
+
+// OptAppendReference sets the reference to be set for the appended item in the
+// RootIndex. The reference is added as an `org.opencontainers.image.ref.name`
+// in the RootIndex.
+func OptAppendReference(r name.Reference) AppendOpt {
+	return func(c *appendOpts) error {
+		c.ref = r
+		return nil
+	}
+}
+
+// AppendImage appends an image to the SIF f, updating the RootIndex to
+// reference it.
+func (f *OCIFileImage) AppendImage(img v1.Image, opts ...AppendOpt) error {
+	return f.append(img, opts...)
+}
+
+// AppendIndex appends an index to the SIF f, updating the RootIndex to
+// reference it.
+func (f *OCIFileImage) AppendIndex(ii v1.ImageIndex, opts ...AppendOpt) error {
+	return f.append(ii, opts...)
+}
+
+func (f *OCIFileImage) append(add mutate.Appendable, opts ...AppendOpt) error {
+	ao := appendOpts{
+		tempDir: os.TempDir(),
+	}
+	for _, opt := range opts {
+		if err := opt(&ao); err != nil {
+			return err
+		}
+	}
+
+	ri, err := f.RootIndex()
+	if err != nil {
+		return err
+	}
+
+	ia := mutate.IndexAddendum{Add: add}
+	if ao.ref != nil {
+		ia.Annotations = map[string]string{
+			"org.opencontainers.image.ref.name": ao.ref.Name(),
+		}
+	}
+	ri = mutate.AppendManifests(ri, ia)
+
+	return f.UpdateRootIndex(ri, OptUpdateTempDir(ao.tempDir))
+}
diff --git a/pkg/sif/update_test.go b/pkg/sif/update_test.go
index 60426de..5bdee6b 100644
--- a/pkg/sif/update_test.go
+++ b/pkg/sif/update_test.go
@@ -9,6 +9,7 @@ import (
 	"os"
 	"testing"
 
+	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/empty"
 	v1mutate "github.com/google/go-containerregistry/pkg/v1/mutate"
@@ -152,3 +153,125 @@ func TestUpdate(t *testing.T) {
 		})
 	}
 }
+
+//nolint:dupl
+func TestAppendImage(t *testing.T) {
+	r := rand.NewSource(randomSeed)
+	newImage, err := random.Image(64, 1, random.WithSource(r))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name string
+		base string
+		opts []sif.AppendOpt
+	}{
+		{
+			name: "Default",
+			base: "hello-world-docker-v2-manifest",
+			opts: []sif.AppendOpt{},
+		},
+		{
+			name: "WithReference", // Replaces many layers with a single layer
+			base: "hello-world-docker-v2-manifest",
+			opts: []sif.AppendOpt{
+				sif.OptAppendReference(name.MustParseReference("myimage:v1", name.WithDefaultRegistry(""))),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sifPath := corpus.SIF(t, tt.base, sif.OptWriteWithSpareDescriptorCapacity(8))
+			fi, err := ssif.LoadContainerFromPath(sifPath)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			ofi, err := sif.FromFileImage(fi)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if err := ofi.AppendImage(newImage, tt.opts...); err != nil {
+				t.Fatal(err)
+			}
+
+			if err := fi.UnloadContainer(); err != nil {
+				t.Fatal(err)
+			}
+
+			b, err := os.ReadFile(sifPath)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			g := goldie.New(t,
+				goldie.WithTestNameForDir(true),
+			)
+
+			g.Assert(t, tt.name, b)
+		})
+	}
+}
+
+//nolint:dupl
+func TestAppendIndex(t *testing.T) {
+	r := rand.NewSource(randomSeed)
+	newIndex, err := random.Index(64, 1, 1, random.WithSource(r))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name string
+		base string
+		opts []sif.AppendOpt
+	}{
+		{
+			name: "Default",
+			base: "hello-world-docker-v2-manifest",
+			opts: []sif.AppendOpt{},
+		},
+		{
+			name: "WithReference", // Replaces many layers with a single layer
+			base: "hello-world-docker-v2-manifest",
+			opts: []sif.AppendOpt{
+				sif.OptAppendReference(name.MustParseReference("myindex:v1", name.WithDefaultRegistry(""))),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sifPath := corpus.SIF(t, tt.base, sif.OptWriteWithSpareDescriptorCapacity(8))
+			fi, err := ssif.LoadContainerFromPath(sifPath)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			ofi, err := sif.FromFileImage(fi)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if err := ofi.AppendIndex(newIndex, tt.opts...); err != nil {
+				t.Fatal(err)
+			}
+
+			if err := fi.UnloadContainer(); err != nil {
+				t.Fatal(err)
+			}
+
+			b, err := os.ReadFile(sifPath)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			g := goldie.New(t,
+				goldie.WithTestNameForDir(true),
+			)
+
+			g.Assert(t, tt.name, b)
+		})
+	}
+}