Li isotopes in compositionally diverse Martinique lavas, as well as sea floor sediments cored at the southern (DSDP Site 144) and northern part (DSDP Site 543) of the subducting slab were analyzed in order to investigate the origin of the continental crust compositional signature seen in Lesser Antilles lavas and to investigate Li cycling in arcs. Although the subducting sediments display marked mineralogical and chemical shifts from south to north, the concentration-weighted mean δ7Li for sediments from the two cores are indiscernible from each other (bulk δ7Li = -0.5 ± 1.8, 1 σ, n = 15, δ7Li = –4.4 to +2.9). This is the lowest bulk δ7Li seen in subducting sediments from any trenches, and is significantly lower than that of MORB (δ7Li ∼ +4 ± 1). These low δ7Li values reflect the dominance of terrigenous input and the influence of chemical weathering in the sediment’s continental provenance. With a few exceptions, the Li isotopic compositions of the Martinique lavas are also systematically lighter than MORB, yielding an average δ7Li of +1.8 ± 1.3 (1 σ, n = 24, excluding three outliers that are isotopically heavy, erupted below seawater and may have incorporated sea water Li). The δ7Li values in the lavas show no correlation with most radiogenic isotope ratios (87Sr/86Sr, 143Nd/144Nd and 176Hf/177Hf), Li/Y, La/Sm or SiO2. There is also no correlation between δ7Li and radiogenic isotopes in the subducting sea floor sediments. Thus, the low δ7Li in the Martinique lavas likely reflects a mantle source that incorporated isotopically light subducted sediments. A two-end-member mixing model requires sedimentary input of <1% to 5% by mass to a depleted mantle source to reproduce the Li isotopic compositions of the mafic samples, consistent with the range of sediment input (0.1–5%) inferred from radiogenic isotope data. The Lesser Antilles is the first arc shown to have δ7Li systematically lower than MORB, reflecting the influence of subducted terrigenous sediments. Our data suggest that the enrichment of Li in most arc lavas is due to the addition of subducted sediment-derived Li, and that the isotopic signature of this Li can traverse the mantle wedge with little modification. It is only in arcs such as the Lesser Antilles where the Li isotopic composition of subducted sediments is very different from MORB that the slab signature is apparent in the lavas.