Update 10/02/2025: As noted by SteamDB on Bluesky, while this specific page on Steam's advertising policy is new, the policy itself is not. Valve has had a ban on these kinds of in-game ads in Steam games for the last five years at minimum, viewable on its pricing policy page.
Original story: First reported by Gaming on Linux, Valve has added a new advertising page to its Steamworks developer documentation. The page outlines what kinds of advertising and promotion it will allow on Steam, while explicitly banning in-game ads as a revenue model on the ubiquitous PC gaming platform.
Product placement is still in, so long as "such portrayals are not disruptive and are appropriate within the context of the game." Valve gives a racing game with real life sponsors on the cars as one example of what's okay, and it sure sounds like my dream reboot of Pepsiman could still find a home on the store, thank god. Cross-promotions are allowed too, but Valve states "under no circumstances is it okay to charge developers to participate in a bundle or sell access to a store page or other page on Steam." Last, paid advertising campaigns outside Steam that lead to a store page are also fair game.
What's been banned is any form of in-game advertising that gates access to, or otherwise interferes with, gameplay. "Developers should not use paid advertising as a business model in their game, such as requiring players to watch or otherwise engage with advertising in order to play," Valve writes on the page. Basically, it sounds like a blanket ban on any sort of "watch this ad to play the game" design, and it also extends to in-game ads being tied to rewards like power ups or in-game currency.
I don't think I've ever encountered or heard of a single game engaging in this behavior on Steam, but given its prevalence on other platforms, particularly mobile games, it makes sense for Valve to lay this out as a more preventative measure. Valve is legendarily light touch when it comes to moderating Steam, but in addition to an "I'll know it when I see it" approach to banning offensive content, Valve has explicitly banned NFT and crypto-derived games, and also requires games relying on generative AI to disclose its use. The company clearly has a sense of what it wants PC gaming to look like as far as its influential platform goes.
