We performed temperature-dependent optical pump–THz emission measurements in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an externally applied magnetic field. We study the temperature dependence of the spin Seebeck effect and observe a continuous increase as temperature is decreased, opposite to what is observed in electrical measurements, where the spin Seebeck effect is suppressed as 0 K is approached. By quantitatively analyzing the different contributions, we isolate the temperature dependence of the spin-mixing conductance and observe features that are correlated with the bands of magnon spectrum in YIG.