There is disclosed a honeycomb structure (1) comprising: a plurality of honeycomb segments (2a, 2b), bonded into one piece, having a large number of cells (6) separated from one another by cell walls (10) and extending through an axial direction. At least one honeycomb segment (2a) not constituting an outermost peripheral surface (23) of the honeycomb structure (1) has a larger average cell wall thickness and a smaller or equal cell density compared with those of at least one honeycomb segment (2b) constituting the outermost peripheral surface (23). There is disclosed a honeycomb structure assembly in which a material B having compressive elasticity is disposed in a compressed state on the outermost peripheral surface (23) of the honeycomb structure (1) to compression-hold the honeycomb structure (1) in a metallic container. The honeycomb structure and assembly can prevent deterioration of reactivity, purification efficiency, and regeneration efficiency at the time of use, and can be low in pressure loss and better in durability against breakage by thermal stress.