A heat exchanger for gases, in particular for exhaust gases from an engine, which comprises: - a tube bundle intended for the circulation of exhaust gases; - a housing which houses within it said tube bundle, extending longitudinally, and which comprises: - an end for the entry of gases and an end for the exit of gases to which are attached corresponding supporting plates (3a, 3b), to which the ends of the tubes (t) in said tube bundle are attached in such a way that the tubes (t) communicate with the exterior of the housing; and - an inlet for cooling fluid and an outlet for cooling fluid to cause a cooling fluid to circulate through the interior of the housing in contact with the tube bundle in order to exchange heat with the exhaust gases circulating within the tubes (t); - a longitudinal deflector located within the housing between the tube bundle and the inlet for cooling fluid which is arranged in such a way as to direct the flow of cooling fluid entering through said cooling fluid inlet to said end for the entry of gases; wherein said tube bundle is located within said housing excentrically with respect to the longitudinal central geometrical axis of the housing, wherein a first portion of the outer perimeter of said tube bundle, which substantially includes all of the outer perimeter less a second portion thereof which is opposite said longitudinal deflector, is located adjacent to and geometrically matches the inner perimeter of the housing in order to prevent the formation of channels for the passage of cooling fluid between the tubes (t) defining said first portion of the outer perimeter of the tube bundle and the housing, wherein the intermediate space between the inner perimeter of the housing and the first portion of the outer perimeter of the tube bundle is substantially constant for all the first portion of the outer perimeter of the tube bundle, the heat exchanger being characterized in that said intermediate space is of a size smaller than or equal to 3 mm in a dimension transverse to any transverse cross-section of the housing, wherein said housing is in the shape of a hollow cylinder, wherein the tubes (t) of the tube bundle are distributed to form at least two rows and wherein said two rows of tubes (t) follow respective curved trajectories between first and second ends thereof, and in that the tube bundle further comprises at least two additional tubes (ta), each of which is located adjacent to the first or second ends of the at least two rows of tubes (t). - a tube bundle intended for the circulation of exhaust gases; - a housing which houses within it said tube bundle, extending longitudinally, and which comprises: - an end for the entry of gases and an end for the exit of gases to which are attached corresponding supporting plates (3a, 3b), to which the ends of the tubes (t) in said tube bundle are attached in such a way that the tubes (t) communicate with the exterior of the housing; and - an inlet for cooling fluid and an outlet for cooling fluid to cause a cooling fluid to circulate through the interior of the housing in contact with the tube bundle in order to exchange heat with the exhaust gases circulating within the tubes (t); - an end for the entry of gases and an end for the exit of gases to which are attached corresponding supporting plates (3a, 3b), to which the ends of the tubes (t) in said tube bundle are attached in such a way that the tubes (t) communicate with the exterior of the housing; and - an inlet for cooling fluid and an outlet for cooling fluid to cause a cooling fluid to circulate through the interior of the housing in contact with the tube bundle in order to exchange heat with the exhaust gases circulating within the tubes (t); - a longitudinal deflector located within the housing between the tube bundle and the inlet for cooling fluid which is arranged in such a way as to direct the flow of cooling fluid entering through said cooling fluid inlet to said end for the entry of gases; wherein said tube bundle is located within said housing excentrically with respect to the longitudinal central geometrical axis of the housing, wherein a first portion of the outer perimeter of said tube bundle, which substantially includes all of the outer perimeter less a second portion thereof which is opposite said longitudinal deflector, is located adjacent to and geometrically matches the inner perimeter of the housing in order to prevent the formation of channels for the passage of cooling fluid between the tubes (t) defining said first portion of the outer perimeter of the tube bundle and the housing, wherein the intermediate space between the inner perimeter of the housing and the first portion of the outer perimeter of the tube bundle is substantially constant for all the first portion of the outer perimeter of the tube bundle, the heat exchanger being characterized in that said intermediate space is of a size smaller than or equal to 3 mm in a dimension transverse to any transverse cross-section of the housing, wherein said housing is in the shape of a hollow cylinder, wherein the tubes (t) of the tube bundle are distributed to form at least two rows and wherein said two rows of tubes (t) follow respective curved trajectories between first and second ends thereof, and in that the tube bundle further comprises at least two additional tubes (ta), each of which is located adjacent to the first or second ends of the at least two rows of tubes (t).