diff --git a/internal/gapicgen/generator/gapics.go b/internal/gapicgen/generator/gapics.go
index d37e98f..15a3373 100644
--- a/internal/gapicgen/generator/gapics.go
+++ b/internal/gapicgen/generator/gapics.go
@@ -146,6 +146,7 @@
 	}
 
 	args := []string{"-I", googleapisDir,
+		"--experimental_allow_proto3_optional",
 		"-I", protoDir,
 		"--go_gapic_out", gocloudDir,
 		"--go_gapic_opt", fmt.Sprintf("go-gapic-package=%s;%s", conf.importPath, conf.pkg),
diff --git a/internal/gapicgen/generator/genproto.go b/internal/gapicgen/generator/genproto.go
index b4d1e9c..e39928e 100644
--- a/internal/gapicgen/generator/genproto.go
+++ b/internal/gapicgen/generator/genproto.go
@@ -195,7 +195,7 @@
 // protoc executes the "protoc" command on files named in fnames, and outputs
 // to "<genprotoDir>/generated".
 func protoc(genprotoDir, googleapisDir, protoDir string, fnames []string) error {
-	args := []string{fmt.Sprintf("--go_out=plugins=grpc:%s/generated", genprotoDir), "-I", googleapisDir, "-I", protoDir}
+	args := []string{"--experimental_allow_proto3_optional", fmt.Sprintf("--go_out=plugins=grpc:%s/generated", genprotoDir), "-I", googleapisDir, "-I", protoDir}
 	args = append(args, fnames...)
 	c := command("protoc", args...)
 	c.Stdout = os.Stdout