We report the first known occurrence of high-Ca boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo86–92 olivines) from a dredged satellite cone have compositions classified as high-Ca boninite. All samples from Volcano A, however, may be related to parental boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high-Ca boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>∼24%) at typical mantle wedge temperatures (<1300°C) driven by an influx of slab-derived fluid (>4 wt % H2O in primary liquids). We propose a two-stage model for generating primary boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back-arc basin, followed by (2) remelting of this residual peridotite with slab-derived fluid beneath the Tonga Arc. The occurrence of high-Ca boninites at Volcano A is related to the relative location and duration of back-arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for ∼1 Ma, and corner flow circulation brings mantle from the back-arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si6.0 and Ti6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back-arc basins, are also erupting basaltic andesites with boninite parentage.