Cooperation in real-world public goods often takes place in multilevel structure and under uncertainty about the returns to collective action. We study how risk in marginal returns effects cooperation in a multilevel public goods game. In an incentivized online experiment, participants are randomly assigned to either a deterministic scenario or an otherwise, identical one with risky marginal! per capita returns, holding expected returns constant. We find that risk reduces total! contributions, mainly by lowering contributions to the local public good. Risk also lowers expectations about locaI cooperation by group members consistent with a conditional-cooperation channel. Moreover, the impact of risk depends on social closeness: individuals who feel closer to their local group tend to reduce global contributions more strongly under risk, while local contributions are not differentially affected.