The advantage of acicular mullite over other ceramic materials is its ability to produce highly porous structures while maintaining high mechanical integrity. This is due to controlled acicular grain growth that affects the connectivity of individual mullite crystals forming a 3D network. High porosity, in turn, enables coating with large amounts of washcoat and catalyst enabling efficient NOx control and other multifunctional applications without penalizing back pressure. This paper describes the development of the experimental ultra‐high porosity acicular mullite ceramic to be used for combined NOx reduction and filtration of soot in diesel engines. Catalyzed acicular mullite honeycombs having porosities between 64% and 80% showed excellent NOx reduction and pressure drop that is comparable with state‐of‐art commercial filters without any catalyst coating. The ability to combine several functions into one substrate provides the promise of a lower cost for the entire emission treatment and creates an opportunity for other applications requiring highly porous but mechanically strong scaffold as a carrier for functional materials.