This paper is concerned with the complexity of computing winning strategies for poset games. While it is reasonably clear that such strategies can be computed in PSPACE, we give a simple proof of this fact by a reduction to the game of geography. We also show how to formalize the reasoning about poset games in Skelley’s theory W1 1 for PSPACE reasoning. We conclude that W1 1 can use the “strategy stealing argument” to prove that in poset games with a supremum the first player always has a winning strategy.