The control of thermal emission is of great importance for emerging applications in energy conversion and thermometric sensing. Usually, thermal emission at ambient temperature is limited to the mid- to far-infrared, according to the linear theory of Planck’s law. We experimentally demonstrate a broadband nonlinear thermal emission in the visible-NIR spectrum within a quadradic nonlinear medium, which emits visible thermal radiation through a pump-driven nonlinear upconversion from its mid-IR components even at room temperature, unlike its linear counterpart which requires ultrahigh temperature. The broadband emission is enabled by the crucial random quasi-phase-matching condition in our nonlinear nanocrystal powders. Moreover, nonlinear thermal emission also permits visible thermometry using traditional optical cameras instead of thermal ones. This scheme paves the way to understand thermal radiation dynamics with nonlinearity in many fields, such as nonlinear heat transfer and nonlinear thermodynamics.