A heat exchanger for a transmission unit of an aircraft, comprising: - a first module defining a first feed path for a first fluid to be cooled; - a second module defining a second feed path for a second, cooling fluid; said first and second feed paths (P, T) being, in use, thermally coupled to each other; each said second module comprising at least one cell formed by: - an inlet for a second, cooling fluid; - at least one outlet for said second cooling fluid, which is arranged on the opposite side to said inlet along a first direction; - at least one first wall thermally coupled to said first path; - a pair of second walls (36a, 36b) transversal to said first wall; - a plurality of fins projecting in a cantilever fashion from said first wall and adapted to increase, in use, the heat exchange between said second cooling fluid and said first wall; and - at least a first row of said fins, which lie on a plane orthogonal to said first direction; said fins of said first row extending at progressively increasing distances from one of said second walls (36a, 36b) along a second direction orthogonal to said first direction, when proceeding along said first direction from said inlet to said at least one outlet; - at least a second row of said fins; said fins of said second row extending at progressively increasing distances from the other of said second walls (36a, 36b) along said second direction, characterized in that said cell comprises at least two said outlets;said first and second row (61, 62) delimiting between them a first chamber delimited by said inlet;one of said second walls (36a, 36b) and said first row defining a second chamber delimited by one of said outlets;the other of said second walls (36a, 36b) and said second row defining a third chamber delimited by the other of said outlets;said first chamber and said second chamber (52, 53) being in fluidic communication with each other via first said passages defined between respective mutually consecutive first fins of said first row;said first chamber and said third chamber (52, 53) being in fluidic communication with each other via second said passages defined between respective mutually consecutive first fins of said second row. - a first module defining a first feed path for a first fluid to be cooled; - a second module defining a second feed path for a second, cooling fluid; said first and second feed paths (P, T) being, in use, thermally coupled to each other; each said second module comprising at least one cell formed by: - an inlet for a second, cooling fluid; - at least one outlet for said second cooling fluid, which is arranged on the opposite side to said inlet along a first direction; - at least one first wall thermally coupled to said first path; - a pair of second walls (36a, 36b) transversal to said first wall; - a plurality of fins projecting in a cantilever fashion from said first wall and adapted to increase, in use, the heat exchange between said second cooling fluid and said first wall; and - at least a first row of said fins, which lie on a plane orthogonal to said first direction; said fins of said first row extending at progressively increasing distances from one of said second walls (36a, 36b) along a second direction orthogonal to said first direction, when proceeding along said first direction from said inlet to said at least one outlet; - an inlet for a second, cooling fluid; - at least one outlet for said second cooling fluid, which is arranged on the opposite side to said inlet along a first direction; - at least one first wall thermally coupled to said first path; - a pair of second walls (36a, 36b) transversal to said first wall; - a plurality of fins projecting in a cantilever fashion from said first wall and adapted to increase, in use, the heat exchange between said second cooling fluid and said first wall; and - at least a first row of said fins, which lie on a plane orthogonal to said first direction; said fins of said first row extending at progressively increasing distances from one of said second walls (36a, 36b) along a second direction orthogonal to said first direction, when proceeding along said first direction from said inlet to said at least one outlet; - at least a second row of said fins; said fins of said second row extending at progressively increasing distances from the other of said second walls (36a, 36b) along said second direction,