[en] Based on first-principles calculations, we study the noncollinear magnetism in the post-perovskites (pPv) phase of NaNiFinline image and CaRhOinline image 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 inline image with a magnetically active B-site. In the pPv phase of NaNiFinline image, which has a inline image antiferromagnetic ground state with a inline image ferromagnetic canting, we obtain a magnetic canting amplitude of about 0.1 inline imageatominline image, which is much larger than in the one obtained in CaRhOinline image (canting amplitude of 0.04 inline image). We also computed the exchange constants (inline image), 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 NaNiFinline image is mainly due to the DM interaction, while in CaRhOinline image, 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.