A heat exchanger for a boiler or similar heating device comprises a casing and a tube assembly inside the casing. The tube assembly includes a plurality of modules (6.sub.x, 6.sub.y) arranged in a juxtaposed configuration, each module (6.sub.x, 6.sub.y) having an at least approximately annular shape. The modules (6.sub.x,6.sub.y) each include a respective tube (7) that is at least partially embedded in a respective thermally conductive body (8) overmoulded to the tube (7). Each thermally conductive body (8) defines an upper face and a lower face of the respective module (6.sub.x, 6.sub.y), where at least at the upper face of a first module (6.sub.y) and the lower face of a second module (6.sub.x) the corresponding thermally conductive body (8) defines upper fins (24) and lower fins (23.sub.x), respectively, which extend in height substantially in an axial direction of the tube assembly and extend in length substantially in a radial direction of the tube assembly (5). The upper fins (24) of the thermally conductive body (8) of the first module (6.sub.y) are in an axially staggered position with respect to the lower fins (23.sub.x) of the thermally conductive body (8) of the second module (6.sub.x), with the upper fins (24), on the one hand, and with the lower fins (23.sub.x), on the other hand, which are at mutual distances such that the upper fins (24) of the thermally conductive body (8) of the first module (6.sub.y) are set between the lower fins (23.sub.x) of the thermally conductive body (8) of the second module (6.sub.x), or vice versa. In this way, between each upper fin (24) of the thermally conductive body (8) of the first module (6.sub.y) and each lower fin (23.sub.x) of the thermally conductive body (8) of the second module (6.sup.x), or vice versa, a respective radial passageway (P) is defined for the combustion fumes produced by a burner equipping the heat exchanger.