Subduction alters continents several ways, including accretion, magmatic addition, mantle wedge serpentinization, and crustal differentiation. These changes affect seismic velocities, so characterizing upper plate crust establishes a baseline for composition and continental growth. Teleseismic P and PP arrivals from a temporary deployment of broadband seismometers in Central America have been used to estimate crustal thickness and Vp/Vs ratio from receiver functions and to image crust across the active arc. Crustal thickness ranges from 25 to 44 km with formal errors of 1.6–9.2 km. The thinnest crust (24.6 ± 3.5 km) lies directly beneath the Nicaraguan arc, whereas the thickest crust lies in the Nicaraguan back arc (43.5 ± 2.5 km) and beneath the Costa Rican arc (37.9 ± 5.2 km). Crustal structure and Vp/Vs show sharp transitions at terrane boundaries. The Moho exhibits strong velocity contrasts throughout the study area of ∼0.5–1.0 km/s, even beneath arc and fore arc, precluding extensive serpentinization or ponded melt below the Moho. Crust is thicker beneath the Costa Rican arc, consistent with 10–23 km3/km/Ma crustal growth. The crust is thinner by 11–18 km beneath the large depression in central Nicaragua, with the thinnest crust beneath the arc. There, the relationship between thin crust, arc location, and deeper seismic velocities suggests that upper plate structure plays a critical role in focusing magma to the surface.