["Oxford Bulletin of Economics and Statistics, EarlyView. ", "\nABSTRACT\nThis study investigates pro‐sociality among individuals during an ongoing war. We focus on Yemen, where the recent large‐scale conflict has divided the country geographically, putting different regions in opposition with one another. We collect original survey data and develop a mobile phone‐based version of a public good game to allow participants from regions on opposite sides of the conflict to play in pairs across the entire country. Participants play two games: one with an in‐group partner (from the same region) and one with a randomly assigned partner from a different region—either from the antagonistic out‐group (opposite conflict side) or from the non‐antagonistic out‐group (same conflict side). We further merge our data with the Armed Conflict Location and Event Data Project (ACLED) conflict data to explore whether conflict intensity affects participants' behaviours in the games. Our results show a lower level of cooperation with the rival out‐group compared to the non‐rival out‐group, along with higher in‐group cooperation among participants heavily exposed to the war. Further results show that lower pro‐sociality towards the rival out‐group is mostly found among players from the North of Yemen, where sectarian identity is most clearly tied to the conflict division, and this effect is exacerbated by the intensity of conflict exposure. These results provide some of the clearest empirical support to date for concerns about the destruction of inter‐group cooperation during an ongoing war.\n"]