A chiral absorber of light can emit spin-polarized (circularly polarized) thermal radiation based on Kirchhoff's law which is applicable only for reciprocal media at thermal equilibrium. No such law exists for nonreciprocal media. Here we prove three spin-resolved Kirchhoff's laws of thermal radiation applicable for reciprocal and nonreciprocal planar media. These laws are applicable to birefringent crystals (uniaxial or biaxial), Weyl semimetals, magnetized insulators, semiconductors and plasmas (gyroelectric/gyromagnetic media) as well as magnetoelectric topological insulators, metamaterials and multi-ferroic media. We further propose an experiment to verify these laws using a single system of doped Indium Antimonide (InSb) thin film in an external magnetic field. Our work highlights the fundamentally intriguing role of photon spin in the context of nonreciprocal media and paves the way for novel practical applications based on nonreciprocal thermal systems.