Third-Party Cookie

A third-party cookie is one set by a domain other than the site you are on, used to follow you from site to site.

These are dropped by scripts belonging to someone other than the site in your address bar, usually ad and tracking networks. Because the same network sits on thousands of sites, its cookie sees you on all of them and stitches your activity together, which is how you get retargeting ads for a product you glanced at somewhere else.

For example, you look at a pair of shoes on one shop, then two hours later the same shoes chase you around a news site. That is a third-party cookie recognizing you across both places and telling the ad network to show the ad.

Here is the fact most guides still get wrong: Google reversed course, so third-party cookies are not being removed from Chrome. In April 2025 Google dropped its plan for a cookie opt-out prompt, and in October 2025 it shut down the Privacy Sandbox project entirely, so as of 2026 these cookies still work in Chrome with no deprecation timeline. Safari and Firefox, though, have blocked them for years, so a large slice of your audience is already invisible to them.

Not dead, just half blind.