Lesser Antilles arc lavas have trace element and radiogenic isotope characteristics indicative of a continent-derived contribution. It is debated vigorously whether this continental signature represents terrigenous sediment that has been subducted with the Atlantic plate and added to the magma sources in the mantle wedge, or portions of the subarc crust that are assimilated during magma ascent. Here we present Mo isotope data for Lesser Antilles arc lavas and sediments offboard the Lesser Antilles trench. Sequences of black shales, present in the subducting sediment piles, are highly enriched in Mo and have unusually high 98Mo/95Mo. Despite their low mass fraction in the sediment package (<10% in Deep Sea Drilling Project Site 144), they dominate the Mo content and isotopic composition of the bulk sediment subducting at the Lesser Antilles trench. We show that lavas from the southern part of the Lesser Antilles arc also have high 98Mo/95Mo ratios, implicating the addition of Mo derived from the subducted black shales to their mantle sources. This establishes a new link between the composition of subducted material and the arc lava output.