We present new field observations and laboratory data confirming the presence of the Mattawa Anorthosite Massif (MAT), whose existence in south-central Quebec was hinted at more than 35 years ago. MAT thus represents a newly recognized member of the late- to post-tectonic ∼1060–1010 Ma andesine anorthosite belt that includes the Château-Richer, St. Urbain, and Labrieville massifs. The dominant rock type at MAT is foliated andesine anorthosite or leuconorite, and orientations of foliations indicate that the pluton has the shape of a dome. MAT contains separate core and border zones, which are distinguished on the basis of plagioclase composition and concentrations of Ga, Rb, Sr, and Ba. Xenoliths of labradorite anorthosite having Ga, Sr, and Ba concentrations different from those of the host andesine anorthosites occur sporadically throughout the pluton as well. Lastly, rocks enriched in Fe, Ti, and P (jotunite, oxide–apatite gabbronorite, nelsonite, ilmenitite) also occur at MAT, primarily near the core–border transition or at the pluton margins. Compared with other anorthosites in the andesine belt, MAT is compositionally most similar to Labrieville. By analogy with Labrieville, we interpret the concentric zoning of MAT (more sodic core, more calcic border) to reflect polybaric crystallization accompanying upward intrusion as a magmatic diapir, which also produced the overall domal structure. The labradorite xenoliths bear little physical or compositional resemblance to typical Lac St. Jean rocks. Therefore, if the xenoliths were derived from the Lac St. Jean Anorthosite, their present character must reflect considerable modification by the Mattawa magma.