Based on first‐principles calculations, we study the noncollinear magnetism in the post‐perovskites (pPv) phase of NaNiF and CaRhO crystals. We find that the magnetic canting is one of the most promising properties of pPv systems, which is allowed by symmetry in all the pPv crystals with a magnetically active B ‐site. In the pPv phase of NaNiF , which has a antiferromagnetic ground state with a ferromagnetic canting, we obtain a magnetic canting amplitude of about 0.1 atom , which is much larger than in the one obtained in CaRhO (canting amplitude of 0.04 ). We also computed the exchange constants ( ), the single‐ion anisotropy (SIA) parameters and the anti‐symmetric magnetic coupling described by the Dzyaloshinsky–Moriya (DM) interaction in order to scrutinize the origin of the magnetic canting. We find that the canting in NaNiF is mainly due to the DM interaction, while in CaRhO , both DM and SIA contribute to the magnetic canting. Our calculations thus confirm the noncollinear magnetic ground‐state solution experimentally observed in both compounds and the calculated magnetic exchange interactions also confirm the quasi‐2D magnetic behavior reported in pPv.