OpenAI taps Magnite and PubMatic to sell ChatGPT's leftover ad space

OpenAI is handing its unsold ChatGPT ad slots to two outside auctioneers. Magnite and PubMatic have both signed on to sell inventory the company hasn't moved through its own direct deals, according to investing.com. Neither side disclosed what the partnerships are worth.

Both firms are sell-side platforms, or SSPs — think of them as automated auction houses for ad space. A publisher feeds in open slots, advertisers bid in real time, and software matches them in milliseconds. That hand-off process is what the industry calls programmatic advertising.

The inventory in question is remnant stock: the slots left over after a publisher's direct sales team is done. How revenue gets split, and how much of ChatGPT's total ad space will actually flow through Magnite and PubMatic, remains unclear because the terms stayed private.

OpenAI's ad push has moved fast. Testing began in the United States in February 2026 with free and Go-tier users, and by March the business was running at roughly $100 million annualized. By August 2026, that pace had climbed to a $1 billion annualized run rate — a projection of recent revenue, not cash already banked. As of October 21, 2026, ChatGPT ads had reached 89 countries and more than 100,000 advertisers.

It's a striking reversal. OpenAI once pitched advertising as a funding option of last resort, then changed course as it hunted for revenue beyond subscriptions. The Magnite and PubMatic tie-ups slot into a wider ad ecosystem the company has been assembling, which already includes agencies and tech platforms — giving buyers multiple doors into ChatGPT. One boundary holds: premium subscription tiers stay ad-free.

For the two ad-tech firms, this is a trophy supply deal. Independent SSPs spend their lives scraping against Google's AdX, and locking in inventory from one of the most watched AI products on the planet is a real win. Advertisers already plugged into these platforms may get a cheaper on-ramp to ChatGPT users instead of building a new relationship from zero. What nobody outside the deal can say yet is how much any of it adds to OpenAI's top line.