diff --git a/README.md b/README.md
index cb273da..e56e8e7 100644
--- a/README.md
+++ b/README.md
@@ -132,6 +132,12 @@ Images are published to [Docker Hub](https://hub.docker.com/r/temporalio/tempora
 docker pull temporalio/temporal-proxy:latest
 ```
 
+To guard against a deployment that would forward payloads unencrypted, start the proxy with
+`--require-encryption` (or set `PROXY_REQUIRE_ENCRYPTION=true`). The proxy then refuses to start unless the config
+has an encryption key (`encryption.default`). It checks for a key rather than `encryption.enabled`, so a decrypt-only
+proxy (a key with `enabled: false`) still starts. The flag sits outside the config file so that editing the config
+alone cannot turn it off.
+
 ### Helm
 
 A chart is published to the Temporal Helm repo at `https://go.temporal.io/helm-charts`:
diff --git a/cmd/proxy/serve.go b/cmd/proxy/serve.go
index e6ac34f..441f462 100644
--- a/cmd/proxy/serve.go
+++ b/cmd/proxy/serve.go
@@ -58,6 +58,11 @@ func serve() *cli.Command {
 				Value:   "info",
 				Sources: cli.EnvVars("LOG_LEVEL"),
 			},
+			&cli.BoolFlag{
+				Name:    "require-encryption",
+				Usage:   "Refuse to start unless the config has an encryption key (encryption.default)",
+				Sources: cli.EnvVars("PROXY_REQUIRE_ENCRYPTION"),
+			},
 		},
 		Action: func(ctx context.Context, cmd *cli.Command) error {
 			log := logger.NewZeroLogger(os.Stderr, logger.ParseLevel(cmd.String("level")))
@@ -81,6 +86,7 @@ func serve() *cli.Command {
 				auth.Module,
 				codecserver.Module,
 				config.Module,
+				config.RequireKey(cmd.Bool("require-encryption")),
 				connect.Module,
 				dataplane.Module,
 				httpserver.Module,
diff --git a/internal/config/encryption.go b/internal/config/encryption.go
index aebaba9..1987243 100644
--- a/internal/config/encryption.go
+++ b/internal/config/encryption.go
@@ -138,6 +138,21 @@ func (e *Encryption) Validate() error {
 	return validation.Validate("", rules...)
 }
 
+// RequireKey fails unless a Default key policy is configured, whether or not
+// encryption is Enabled.
+func (e *Encryption) RequireKey() error {
+	return validation.Validate(
+		"encryption",
+		validation.Field("default", e.Default, func(p *KeyPolicy) error {
+			if p == nil {
+				return errors.New("a key is required when encryption is required")
+			}
+
+			return nil
+		}),
+	)
+}
+
 // Mode maps b to the vault's binding mode. An absent value is an error rather
 // than a default, since the right mode depends on whether earlier payloads exist.
 // It is the only place the spellings are known, so validation and vault
diff --git a/internal/config/fx.go b/internal/config/fx.go
index 5bc579f..ec84637 100644
--- a/internal/config/fx.go
+++ b/internal/config/fx.go
@@ -38,3 +38,16 @@ func Supply(cfg *Config) fx.Option {
 		NewAllowlist,
 	)
 }
+
+// RequireKey fails the graph unless the provided *Config has an encryption key
+// (see [Encryption.RequireKey]). When required is false it adds nothing. It
+// works with either [Module] or [Supply].
+func RequireKey(required bool) fx.Option {
+	if !required {
+		return fx.Options()
+	}
+
+	return fx.Invoke(func(cfg *Config) error {
+		return cfg.Encryption.RequireKey()
+	})
+}
diff --git a/internal/config/fx_test.go b/internal/config/fx_test.go
index 61cc1bf..347e52b 100644
--- a/internal/config/fx_test.go
+++ b/internal/config/fx_test.go
@@ -135,3 +135,49 @@ func TestModuleRejectsInvalidFile(t *testing.T) {
 	)
 	require.ErrorContains(t, app.Err(), "at least one upstream is required")
 }
+
+// TestRequireKey covers the startup guard against a proxy that would forward
+// plaintext. It keys on a configured key, not on enabled, so a decrypt-only
+// proxy (a key with enabled false) still starts.
+func TestRequireKey(t *testing.T) {
+	t.Parallel()
+
+	const key = "  namespaceBinding: strict\n  default:\n    uri: testing://a2V5\n    duration: 1h\n    renewBefore: 10m\n"
+
+	tests := []struct {
+		name    string
+		require bool
+		yaml    string
+		wantErr bool
+	}{
+		{name: "no encryption block", require: true, yaml: "", wantErr: true},
+		{name: "disabled without a key", require: true, yaml: "encryption:\n  enabled: false\n", wantErr: true},
+		{name: "decrypt-only with a key", require: true, yaml: "encryption:\n  enabled: false\n" + key},
+		{name: "enabled with a key", require: true, yaml: "encryption:\n  enabled: true\n" + key},
+		{name: "not required and no key", require: false, yaml: ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			path := filepath.Join(t.TempDir(), "config.yaml")
+			require.NoError(t, os.WriteFile(path, []byte("hostPort: :7233\n"+upstreamYAML+tt.yaml), 0o600))
+
+			app := fx.New(
+				fx.NopLogger,
+				fx.Supply(fx.Annotate(path, config.ConfigFileTag)),
+				config.Module,
+				config.RequireKey(tt.require),
+				fx.Invoke(func(*config.Config) {}),
+			)
+
+			if tt.wantErr {
+				require.ErrorContains(t, app.Err(), "encryption: default: a key is required when encryption is required")
+				return
+			}
+
+			require.NoError(t, app.Err())
+		})
+	}
+}