Volcanic outputs (ash, tephra, and lava samples) of 23 volcanic centers from the volcanic front, secondary front, and back arc in Central America were analyzed for their δ37Cl values with the goal of using chlorine isotopes as a tracer of fluids sourced from the subducting Cocos slab. δ37Cl values range from −2.6 to +3.0‰ with systematic variations along the length of the front. Values from the northernmost (Guatemala and El Salvador) and southernmost (Costa Rica) ends of the front have mantle-like signatures. In contrast, δ37Cl values are both positive and negative in the center of the front (Nicaragua), implying a sediment and/or serpentinite-derived component. Geophysical observations are consistent with extensive hydration of the Cocos plate offshore of Nicaragua, in support of a serpentinite-derived fluid source. Fluids from dehydrating serpentinites may also incorporate Cl from overlying sediments, resulting in a multiple-source chlorine signature.