diff --git a/common/changes/@snowplow/browser-tracker-core/feature-set_network_user_id_2023-10-09-16-16.json b/common/changes/@snowplow/browser-tracker-core/feature-set_network_user_id_2023-10-09-16-16.json
new file mode 100644
index 000000000..4d5407c0f
--- /dev/null
+++ b/common/changes/@snowplow/browser-tracker-core/feature-set_network_user_id_2023-10-09-16-16.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@snowplow/browser-tracker-core",
+      "comment": "Add setNetworkUserId() function",
+      "type": "none"
+    }
+  ],
+  "packageName": "@snowplow/browser-tracker-core"
+}
\ No newline at end of file
diff --git a/common/changes/@snowplow/browser-tracker/feature-set_network_user_id_2023-10-09-16-16.json b/common/changes/@snowplow/browser-tracker/feature-set_network_user_id_2023-10-09-16-16.json
new file mode 100644
index 000000000..a18df5682
--- /dev/null
+++ b/common/changes/@snowplow/browser-tracker/feature-set_network_user_id_2023-10-09-16-16.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@snowplow/browser-tracker",
+      "comment": "Add setNetworkUserId() function",
+      "type": "none"
+    }
+  ],
+  "packageName": "@snowplow/browser-tracker"
+}
\ No newline at end of file
diff --git a/libraries/browser-tracker-core/src/tracker/index.ts b/libraries/browser-tracker-core/src/tracker/index.ts
index a6443a932..9f70bbd84 100755
--- a/libraries/browser-tracker-core/src/tracker/index.ts
+++ b/libraries/browser-tracker-core/src/tracker/index.ts
@@ -318,7 +318,8 @@ export function Tracker(
       configSessionContext = trackerConfiguration.contexts?.session ?? false,
       toOptoutByCookie: string | boolean,
       onSessionUpdateCallback = trackerConfiguration.onSessionUpdateCallback,
-      manualSessionUpdateCalled = false;
+      manualSessionUpdateCalled = false,
+      networkUserId: string | undefined;
 
     if (trackerConfiguration.hasOwnProperty('discoverRootDomain') && trackerConfiguration.discoverRootDomain) {
       configCookieDomain = findRootDomain(configCookieSameSite, configCookieSecure);
@@ -336,6 +337,7 @@ export function Tracker(
     core.addPayloadPair('lang', browserLanguage);
     core.addPayloadPair('res', resolution);
     core.addPayloadPair('cd', colorDepth);
+    setNetworkUserId(networkUserId);
 
     /*
      * Initialize tracker
@@ -1149,6 +1151,11 @@ export function Tracker(
       activityTrackingConfig.configurations[actionKey] = undefined;
     }
 
+    function setNetworkUserId(networkUserId: string | undefined): void {
+      if (networkUserId) {
+        core.addPayloadPair('tnuid', networkUserId);
+      }
+    }
     const apiMethods = {
       getDomainSessionIndex: function () {
         return memorizedVisitCount;
@@ -1311,6 +1318,8 @@ export function Tracker(
       },
 
       clearUserData: clearUserDataAndCookies,
+
+      setNetworkUserId,
     };
 
     return {
diff --git a/libraries/browser-tracker-core/src/tracker/types.ts b/libraries/browser-tracker-core/src/tracker/types.ts
index 0f5801c9d..dbc313609 100755
--- a/libraries/browser-tracker-core/src/tracker/types.ts
+++ b/libraries/browser-tracker-core/src/tracker/types.ts
@@ -602,6 +602,12 @@ export interface BrowserTracker {
    * @param configuration - The plugin to add
    */
   addPlugin: (configuration: BrowserPluginConfiguration) => void;
+
+  /**
+   * Set the newtork user id
+   * @param networkUserId - The custom network user id
+   */
+  setNetworkUserId: (networkUserId: string) => void;
 }
 
 /**
diff --git a/trackers/browser-tracker/src/api.ts b/trackers/browser-tracker/src/api.ts
index e8eba06b6..b5d3c9a99 100644
--- a/trackers/browser-tracker/src/api.ts
+++ b/trackers/browser-tracker/src/api.ts
@@ -506,3 +506,15 @@ export function addPlugin(configuration: BrowserPluginConfiguration, trackers?:
     t.addPlugin(configuration);
   });
 }
+
+/**
+ * Set the business-defined network user ID for this user.
+ *
+ * @param networkUserId - The business-defined network user ID
+ * @param trackers - The tracker identifiers which will be configured
+ */
+export function setNetworkUserId(networkUserId: string, trackers?: Array<string>) {
+  dispatchToTrackers(trackers, (t) => {
+    t.setNetworkUserId(networkUserId);
+  });
+}