The Cauchy horizon in the interior of a Reissner–Nordström black hole (extremal or subextremal) is subject to an infinite blueshift instability. In 1989, Poisson and Israel discovered a nonlinear manifestation of this instability in the spherically symmetric and subextremal setting called ""mass inflation,"" by which the Hawking mass becomes identically infinite at the Cauchy horizon. As Gajic–Luk showed, mass inflation does not occur in the extremal setting. We complete the first proof of mass inflation in the subextremal setting for a wave-type matter model, namely the spherically symmetric Einstein–Maxwell–(uncharged) scalar field system. This result follows from a large-data decay result for the scalar field in the black hole exterior combined with works of Dafermos, Luk–Oh, and Luk–Oh–Shlapentokh-Rothman.