Because of rapid solidification involved in the laser or e-beam based additive manufacturing (AM) process, solution treatable metallic parts made by these methods usually possess a unique nonequilibrium microstructure which changes significantly during subsequent thermal treatment. Such evolution alters the size, morphology, length scale, and distribution of microstructural features and has a substantial impact on thermal properties and possibly on electrical properties as well. This study focuses on effects of microstructural evolution on thermal properties of an additively manufactured AlSi10Mg part. The changes of thermal properties such as thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity as a function of thermal treatment are reported. The results show that the formation of supersaturated primary α aluminum and unique cellular structure imparted by fast solidification in the AM process are the major cause for the low thermal diffusivity and low thermal conductivity observed in this solution treatable, as-built part. A correlation between microstructural evolution and changes in thermal properties is established. Advantages and tailoring of the thermal properties of additively built parts are discussed. Implications of these results are important for other additively manufactured components based on popular solution treatable alloys.