How do rising powers choose to allocate their finite resources among the multiple global and regional security organizations? Building on the literatures on forum shopping and rising powers, we argue that the different organizational investment choices of rising powers are explained by varying regional ideational affinities. Organizational settings have ideational foundations that can look very different from region to region. We argue that regional ideational affinity leads rising powers to invest in regional rather than global organizations. However, if the ideational composition of the region is highly diverse, global organizations are a better vehicle to accommodate rising powers’ emergent ambitions. To demonstrate our argument, we examine the choices of Brazil and South Africa in terms of their material and ideational investments in regional and global organizations.