The thermal response of epitaxial Bi films deposited on Si(001) upon femtosecond laser pulse excitation is investigated by means of ultrafast electron diffraction. The initial surface temperature increase is caused by linear absorption of 800 nm photons. The exponential decay of the transient film temperature is governed by the thermal boundary conductance of the Bi-Si interface. The decay constant linearly depends on the film thickness and was found between 550 and 1100 ps in the thickness range from 6 to 12.2 nm. The thermal boundary conductance of the Bi-Si interface extracted from the linear dependence yields σK=(1320±60) W∕(cm2 K) in accordance with earlier calculations.