Good thermoelectric materials require a small thermal conductivity while maintaining a high electrical conductivity and Seebeck coefficient. Nano-porous systems provide one possible route of increasing the thermoelectric figure-of-merit by disturbing phonon transport more than electron transport. In this work, the temperature dependent thermal conductivity of nano-porous bismuth thin films was measured. Thin Bi films of various porosity and thickness were deposited by metal-organic deposition onto Si substrates. The thermal conductivity of Bi films was measured using a differential 3-ω method, in which the temperature rise across a Bi film was experimentally measured and used to calculate its thermal conductivity. Experimental results show an order-of-magnitude reduction in the porous Bi thin film thermal conductivity due to nano-pores compared to non-porous films. The drastic reduction in thermal conductivity cannot be explained by available models on the thermal conductivity of porous media.