AbstractThermal nonreciprocity typically stems from nonlinearity or spatiotemporal variation of parameters. However, constrained by the inherent temperature‐dependent properties and the law of mass conservation, previous works have been compelled to treat dynamic and steady‐state cases separately. Here, by establishing a unified thermal scattering theory, the creation of a convection‐based thermal metadevice which supports both dynamic and steady‐state nonreciprocal heat circulation is reported. The nontrivial dependence between the nonreciprocal resonance peaks and the dynamic parameters is observed and the unique nonreciprocal mechanism of multiple scattering is revealed at steady state. This mechanism enables thermal nonreciprocity in the initially quasi‐symmetric scattering matrix of the three‐port metadevice and has been experimentally validated with a significant isolation ratio of heat fluxes. The findings establish a framework for thermal nonreciprocity that can be smoothly modulated for dynamic and steady‐state heat signals, it may also offer insight into other heat‐transfer‐related problems or even other fields such as acoustics and mechanics.