<p>Rising political animosity threatens democracy in the United States and other nations. Previous research indicates that intergroup contact under favourable conditions can reduce animosity. Other research indicates that mutually beneficial cooperation is essential for maintaining complex social structures. Building on these ideas, we asked whether mutually beneficial cooperation can reduce animosity between opposing political party members and whether this is possible in an anonymous online context. We created an online quiz game, Tango (letstango.org), where Republicans and Democrats partner and communicate in real time. Across five experiments (<i>N</i> = 4,493, four preregistered), we find that an hour of gameplay with an outparty partner can reduce negative partisanship, with increased self-reported warmth, more equitable economic allocations and more favourable outparty meta-perceptions persisting for up to four months. Gameplay also improves democracy-related attitudes, with effects persisting for one week. The game receives high enjoyability ratings, which may increase motivation to engage with this intervention.</p>