diff --git a/core/packages/gax/src/clientInterface.ts b/core/packages/gax/src/clientInterface.ts
index bf4571d6268..ab0dd2f1543 100644
--- a/core/packages/gax/src/clientInterface.ts
+++ b/core/packages/gax/src/clientInterface.ts
@@ -42,6 +42,9 @@ export interface ClientOptions
   universe_domain?: string;
   /**
    * Whether to enable telemetry tracing for the client.
+   *
+   * The `GOOGLE_SDK_NODE_ENABLE_TRACING` environment variable overrides this
+   * option whenever it is set.
    */
   enableTelemetryTracing?: boolean;
 }
diff --git a/core/packages/gax/src/util.ts b/core/packages/gax/src/util.ts
index de388fe4c83..893ff766880 100644
--- a/core/packages/gax/src/util.ts
+++ b/core/packages/gax/src/util.ts
@@ -23,16 +23,45 @@ const randomUUID = () =>
   globalThis.crypto?.randomUUID() || require('crypto').randomUUID();
 
 /**
- * Checks if telemetry tracing is enabled
+ * Checks if telemetry tracing is enabled.
+ *
+ * Tracing is opt-in, and a caller opts in either with the
+ * `enableTelemetryTracing` client option or with the
+ * `GOOGLE_SDK_NODE_ENABLE_TRACING` environment variable. The environment
+ * variable wins whenever it is set, so tracing can be switched on or off for a
+ * process without touching the code that constructs the client.
+ *
+ * Two further conditions apply while the feature is experimental:
+ * `GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED` must be `true`, and the client
+ * must have supplied `internalTelemetryInfo` — a client generated without
+ * tracing has no span metadata to report, so there is nothing to trace.
+ *
  * @param settings
  * @returns true if telemetry tracing is enabled, false otherwise
  */
 export function checkTelemetryEnabled(settings?: CallSettings): boolean {
-  const tracingEnabled =
-    Boolean(settings?.enableTelemetryTracing) &&
-    process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED === 'true' &&
-    settings?.otherArgs?.internalTelemetryInfo !== undefined;
-  return Boolean(tracingEnabled);
+  // `process` is undeclared in browsers and some edge runtimes, where reading
+  // it would throw a ReferenceError rather than yield undefined, so it is
+  // reached through a `typeof` guard and stands in as an empty environment.
+  // Tracing is then simply off there, since the environment cannot opt in.
+  const env: Record<string, string | undefined> =
+    typeof process === 'object' && typeof process.env === 'object'
+      ? process.env
+      : {};
+
+  // An absent or empty environment variable counts as unset, which is what
+  // separates it from an explicit `false`: only an explicit value overrides
+  // the opt-in the caller passed in `clientOptions`.
+  const envOptIn = env.GOOGLE_SDK_NODE_ENABLE_TRACING?.trim();
+  const tracingRequested = envOptIn
+    ? envOptIn.toLowerCase() === 'true'
+    : Boolean(settings?.enableTelemetryTracing);
+
+  return (
+    tracingRequested &&
+    env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED === 'true' &&
+    settings?.otherArgs?.internalTelemetryInfo !== undefined
+  );
 }
 
 function words(str: string, normalize = false) {
diff --git a/core/packages/gax/test/unit/util.ts b/core/packages/gax/test/unit/util.ts
index 89659ac3a47..ae93a29e08b 100644
--- a/core/packages/gax/test/unit/util.ts
+++ b/core/packages/gax/test/unit/util.ts
@@ -203,6 +203,7 @@ describe('util.ts', () => {
   describe('checkTelemetryEnabled', () => {
     afterEach(() => {
       delete process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED;
+      delete process.env.GOOGLE_SDK_NODE_ENABLE_TRACING;
     });
 
     const mockTelemetryInfo: StaticTraceContext = {
@@ -257,5 +258,75 @@ describe('util.ts', () => {
       process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
       assert.strictEqual(checkTelemetryEnabled(undefined), false);
     });
+
+    it('returns true when GOOGLE_SDK_NODE_ENABLE_TRACING=true and the client option is not set', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'true';
+      const noOptInSettings = new CallSettings({
+        otherArgs: {
+          internalTelemetryInfo: mockTelemetryInfo,
+        },
+      });
+      assert.strictEqual(checkTelemetryEnabled(noOptInSettings), true);
+    });
+
+    it('accepts GOOGLE_SDK_NODE_ENABLE_TRACING case-insensitively', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'TRUE';
+      const noOptInSettings = new CallSettings({
+        otherArgs: {
+          internalTelemetryInfo: mockTelemetryInfo,
+        },
+      });
+      assert.strictEqual(checkTelemetryEnabled(noOptInSettings), true);
+    });
+
+    it('returns false when GOOGLE_SDK_NODE_ENABLE_TRACING=false overrides the client option', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'false';
+      assert.strictEqual(checkTelemetryEnabled(mockSettings), false);
+    });
+
+    it('falls back to the client option when GOOGLE_SDK_NODE_ENABLE_TRACING is empty', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = '';
+      assert.strictEqual(checkTelemetryEnabled(mockSettings), true);
+    });
+
+    it('treats an unrecognized GOOGLE_SDK_NODE_ENABLE_TRACING value as disabled', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'yes';
+      assert.strictEqual(checkTelemetryEnabled(mockSettings), false);
+    });
+
+    it('returns false when GOOGLE_SDK_NODE_ENABLE_TRACING=true but the experimental flag is unset', () => {
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'true';
+      assert.strictEqual(checkTelemetryEnabled(mockSettings), false);
+    });
+
+    it('returns false when GOOGLE_SDK_NODE_ENABLE_TRACING=true but internalTelemetryInfo is not set', () => {
+      process.env.GOOGLE_SDK_NODE_EXPERIMENTAL_O11Y_ENABLED = 'true';
+      process.env.GOOGLE_SDK_NODE_ENABLE_TRACING = 'true';
+      assert.strictEqual(checkTelemetryEnabled(new CallSettings({})), false);
+    });
+
+    it('does not throw when process is undefined, as in a browser', () => {
+      const globals = globalThis as {process?: NodeJS.Process};
+      const originalProcess = globals.process;
+      // The result is captured and `process` restored before asserting,
+      // because mocha and assert need `process` themselves.
+      let result: boolean | undefined;
+      let thrown: unknown;
+      delete globals.process;
+      try {
+        result = checkTelemetryEnabled(mockSettings);
+      } catch (e) {
+        thrown = e;
+      } finally {
+        globals.process = originalProcess;
+      }
+      assert.strictEqual(thrown, undefined);
+      assert.strictEqual(result, false);
+    });
   });
 });