The free version is the Complete Edition of Horizon, which includes the Frozen Wilds expansion, various weapon and outfit packs, a digital art book and PS4 theme.
PS4 and PS5 console owners can get it free until May 14 at 8pm PT (May 15 at 4am BST/5am CEST).
Play At Home was revealed in April 2020, when Sony said it had been designed to promote social distancing and aid indie game developers impacted by the coronavirus pandemic.
Uncharted: The Nathan Drake Collection and Journey were given away free at the time, while a $10 million indie support fund was announced to help struggling creators.
Play At Home 2021 will wrap up by offering players free in-game content or virtual currency for 10 games: Call of Duty Warzone, Rocket League, Brawlhalla, MLB The Show 21, NBA 2K21, Destruction AllStars, Rogue Company World of Tanks, World of Warships and Warframe.
Prior to the sequel’s announcement, VGC revealed that PlayStation and Guerrilla are planning a Horizon Zero Dawn trilogy.