Subduction zone magmas are characterized by high concentrations of H2O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H2O concentration of the mantle (H2Oo) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO2 as a proxy for F, then use F to calculateH2Oo from measured H2O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H2O or more in their mantle sources and define positive, approximately linear correlations between H2Oo and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H2Oo). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H2Oo are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H2O. These results contrast with those from ridges, which record low H2Oo (<0.05 wt %) and broadly negative correlations between H2Oo and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H2O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.