A unit (1, 1′) for recovering and converting thermal energy from the exhaust gases of an internal combustion engine (14) of a vehicle comprises a heat exchanger (2, 2′) to be traversed by exhaust gases flowing along a by-pass path (5,5′) branching out from an exhaust gas main line (4) of said internal combustion engine (14) and valve means (12) for controlling the flow of the exhaust gases through said path, said valve means (12) being driven by an actuator device (12A). The by-pass path (5, 5′) is a U-shaped path defined entirely within the heat exchanger (2, 2′),starting from an inlet section (6, 6′) and ending at an outlet section of the heat exchanger, the inlet and outlet sections (7,7′) being located on a same side of the heat exchanger (2) and both opening on an interface conduit portion (3) interposed in said exhaust gas main line (4). The heat exchanger (2, 2′) is arranged so that said U-shaped path is oriented transversely to the direction of the exhaust gas main line (4), in such a manner that the exhaust gases traversing the heat exchanger flow firstly in the transverse direction away from the exhaust gas main line (4) and then back in the transverse direction towards the exhaust gas main line (4), The valve means (12) are arranged within said interface conduit portion (3), between merging points of the inlet and outlet portions (6,7) of the heat exchanger (2,2′).