AbstractPolicy scholars have increasingly focused on collaborative and competitive relationships between stakeholder coalitions. The Advocacy Coalition Framework (ACF) in particular has directed scholarly attention toward such relationships. The ACF defines advocacy coalitions as groups of actors who share beliefs and coordinate their action. However, previous research has been inconsistent in defining and measuring coalitions, which has hampered comparative research and theory building. We present a method called the Advocacy Coalition Index, which measures belief similarity and the coordination of action in a manner that makes it possible to assess the extent to which advocacy coalitions are found in policy subsystems, whether subgroups resemble coalitions, and how individual actors contribute to coalition formation. The index provides a standardized method for identifying coalitions that can be applied to comparative research. To illustrate the effectiveness of the index, we analyze two climate change policy subsystems, namely Finland and Sweden, which have been shown to differ in terms of the association of belief similarity with coordination. We demonstrate that the index performs well in identifying the different types of subsystems, coalitions, and actors that contribute the most to coalition formation, as well as those involved in cross‐coalition brokerage.