diff --git a/pkg/plugin/signature.go b/pkg/plugin/signature.go
index b00d335..e809b5b 100644
--- a/pkg/plugin/signature.go
+++ b/pkg/plugin/signature.go
@@ -46,6 +46,13 @@ var trustedRootFetcher = func() (root.TrustedMaterial, error) {
 // inclusion and a trusted timestamp. Any failure — bad signature, wrong identity,
 // missing log entry, digest mismatch against manifestData — is an error.
 func verifyManifestSignature(sigBundleJSON, manifestData []byte) error {
+	// cosign's LEGACY bundle ({base64Signature,cert,rekorBundle}) is not the
+	// Sigstore protobuf bundle sigstore-go parses; detect it and give a clear
+	// error rather than a cryptic protobuf "unknown field" message. Releases must
+	// sign with `cosign sign-blob --new-bundle-format`.
+	if bytes.Contains(sigBundleJSON, []byte(`"base64Signature"`)) {
+		return fmt.Errorf("signature is in cosign's legacy bundle format; the release must be signed with --new-bundle-format")
+	}
 	var b bundle.Bundle
 	if err := b.UnmarshalJSON(sigBundleJSON); err != nil {
 		return fmt.Errorf("parse signature bundle: %w", err)
diff --git a/pkg/plugin/signature_test.go b/pkg/plugin/signature_test.go
index 1b9f6e6..cb5091d 100644
--- a/pkg/plugin/signature_test.go
+++ b/pkg/plugin/signature_test.go
@@ -1,6 +1,7 @@
 package plugin
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/sigstore/sigstore-go/pkg/root"
@@ -68,6 +69,20 @@ func TestVerifySignedEntity(t *testing.T) {
 	})
 }
 
+// TestVerifyManifestSignature_RejectsLegacyBundle ensures a cosign legacy bundle
+// (the default `cosign sign-blob --bundle` output) is rejected with a clear,
+// actionable error rather than a cryptic protobuf parse failure.
+func TestVerifyManifestSignature_RejectsLegacyBundle(t *testing.T) {
+	legacy := []byte(`{"base64Signature":"MEUCIQ...","cert":"LS0tLS1CRUdJTi...","rekorBundle":{}}`)
+	err := verifyManifestSignature(legacy, []byte(`{"any":"manifest"}`))
+	if err == nil {
+		t.Fatal("expected an error for a legacy cosign bundle, got nil")
+	}
+	if !strings.Contains(err.Error(), "legacy bundle format") {
+		t.Errorf("error %q does not explain the legacy-format problem", err)
+	}
+}
+
 // TestTrustedRootFetcherOverride confirms the fetcher hook is overridable, so
 // verifyManifestSignature never touches the network in tests. (The JSON-bundle
 // unmarshal is sigstore-go's own tested code; the verification policy it feeds