GA4 Cross-Domain Tracking: The Complete Setup Guide (2026)

When do I need GA4 cross-domain tracking?

Cross-domain tracking is needed when a single user journey spans two or more different registered domains and you want GA4 to treat them as a continuous session rather than separate sessions. Common scenarios requiring it: (1) Separate checkout domain — main site at yourdomain.com and checkout at checkout.yourdomain.com (note: subdomains do NOT need cross-domain tracking — they share the GA4 cookie automatically; only different registered domains need it); (2) Third-party checkout — Shopify site at yourshop.com redirects to yourshop.myshopify.com for checkout; (3) Embedded booking tool — main site embeds a booking widget that opens in a new domain (e.g., book.example-hotel.com); (4) Multi-brand property — single GA4 property tracks brand1.com and brand2.com together. The symptom of missing cross-domain tracking: the second domain shows high Direct traffic and incorrectly inflated new user counts, because each arrival from the first domain looks like a new session.

The two configuration steps

Step 1 — Configure your domains in GA4 data stream settings

Where: Admin → Data Streams → select your web stream → Configure tag settings → More settings → Configure your domains

Add all domains that should be part of the same user journey:

• yourdomain.com
• checkoutdomain.com
• Any other domain that users traverse

What this does: Tells GA4's tag to look for the _gl linker parameter when a user arrives from a listed domain, and to write it to the GA4 cookie rather than starting a new session.

Step 2 — Ensure GA4 tag is present on ALL domains

Every domain in the cross-domain configuration must have the GA4 tag (with the same G-XXXXXXXX Measurement ID) installed. If the second domain doesn't have GA4 installed, cross-domain tracking cannot work regardless of configuration.

Same Measurement ID: All domains must use the same GA4 Measurement ID (G-XXXXXXXX). Using different Measurement IDs on different domains means they're tracking separate properties — cross-domain linking doesn't apply.

How the linker parameter works

When a user clicks a link from yourdomain.com to checkoutdomain.com, GA4 automatically appends a _gl parameter to the URL:

The _gl parameter encodes the user's current GA4 client_id. When GA4 loads on checkoutdomain.com, it reads the _gl parameter and sets the _ga cookie to the same client_id that was active on the first domain. This makes GA4 treat the user as the same person continuing the same session.

Important: This only works for link clicks. If the user navigates via:

• Redirect (302 redirect from domain A to domain B) — linker parameter may not be appended
• JavaScript window.location redirect — linker may not be appended
• Form POST to a different domain — linker is not appended to POST requests

For redirects and form POSTs, additional configuration is needed (see below).

Handling form POST cross-domain (checkout forms)

For e-commerce checkouts that POST to a different domain:

Method 1 — Decorate the form action URL GA4's tag can be configured to decorate both link clicks and form submissions:

In GTM → GA4 Configuration tag → Fields to set:

• Field name: linker_param — this exposes the linker parameter as a JavaScript variable you can append to form action URLs

Or use GTM's "Cross-domain measurement" built-in in the GA4 Configuration tag:

• Tag configuration → Advanced settings → Cross-domain measurement → select "Decorate forms"

Method 2 — Use a redirect approach that preserves the _gl parameter Configure your redirect to preserve query parameters: instead of redirecting to https://checkoutdomain.com/checkout, redirect to https://checkoutdomain.com/checkout?_gl={{gl_value}} with the linker value included.

Configuring referral exclusions for cross-domain

Without referral exclusions, when a user returns from checkoutdomain.com back to yourdomain.com, GA4 sees a referral from checkoutdomain.com and starts a new session (attributing the return visit to the checkout domain as a referrer).

Fix: Add the checkout domain to the referral exclusion list.

Admin → Data Streams → Web stream → More tagging settings → Configure your domains (the same place as step 1)

Adding a domain to "Configure your domains" automatically excludes it from referral tracking. You do not need to separately add it to a referral exclusion list — the domains list serves double duty.

Verifying cross-domain tracking is working

Method 1 — Check Direct traffic on checkout domain

Before cross-domain setup: checkout domain shows 40–60% Direct traffic (users arriving from the main site all appear as Direct).

After cross-domain setup: Direct traffic on checkout domain should drop significantly. Remaining Direct will be genuine direct navigation (bookmarked checkout URLs, returning customers).

Method 2 — Inspect the _gl parameter in URLs

Browse your main site, hover over a link to the checkout domain (or click it and check the URL in the browser). The URL should contain ?_gl= parameter.

If _gl is absent: cross-domain configuration isn't applied to that link. Check that both domains are listed in "Configure your domains."

Method 3 — GTM Preview Mode + Network requests

In GTM Preview Mode: navigate from main site to checkout domain. Check the GA4 hit on the checkout domain's page view — the cid (client_id) should match the cid from the main site's hits.