The proliferation and scope expansion of regional organizations (ROs) is one of the most prominent features in contemporary international politics. In particular, many regional economic organizations (REOs) have expanded into the security realm. This trend has often resulted in an overlap with regional security organizations (RSOs) already in place. This study sheds light on this phenomenon by identifying the conditions under which REOs trespass into the security policy domain despite the fact that preexisting RSOs already fulfill security functions. We argue that the presence of strategic rivalries is an important driver of the creation and depth of organizational overlap through scope expansion. Specifically, RSOs that include significant interstate rivalries propel a subgroup of like-minded states to advance and deepen security cooperation through their existing REOs. Using an original data set of security cooperation within economic and security ROs and a quantitative analysis, we find substantial support for this argument.