Open‐celled aluminum nitride ceramic foams were prepared by the polymer sponge replication technique involving aqueous dispersions of passivated AlN. The amount of the Y2O3 and Dy2O3 as sintering aid was varied, and the effects on the densification, microstructure formation, phase composition, and finally, the thermal conductivity were investigated. A typical thermal conductivity of 1.1 W m−1 K−1 was determined for foams at a porosity level of 94.3 vol.%, on average. This measured foam thermal conductivity was subsequently modeled using different porosity ↔ thermal conductivity relations considering the different hierarchical levels of porosity in these foams. From these models, the thermal conductivity of the bulk AlN strut material was determined, correlated with the strut microstructure and the phase composition, and compared to literature data.