An exhaust gas treatment device for an exhaust system of an internal combustion engine; the treatment device comprises: a tubular duct, which is delimited by a tubular side wall and has an exhaust gas inlet opening, an exhaust gas outlet opening opposite the inlet opening, and an injection opening, which is obtained through the tubular side wall between the inlet opening and the outlet opening and is designed to receive an injector for a reducing substance; and a mixing body, which is arranged inside the tubular duct and creates a forced path to be followed by the exhaust gases in order to flow from the inlet opening to the outlet opening; wherein the mixing body comprises a first base wall with a partially circular shape, which faces the outlet opening of the tubular duct and partially engages the cross section of the tubular duct leaving free a passage area; wherein the mixing body comprises a second base wall with a circular shape, which faces the inlet opening of the tubular duct, completely engages the cross section of the tubular duct and has, at the centre, a main hole, which is coaxial to the tubular duct and is a through hole in order to allow for the passage of the exhaust gases; and wherein the mixing body comprises a first deflector wall, which extends from the first base wall towards the second base wall; the treatment device is characterized in that: the first deflector wall forms an acute angle with the first base wall and with the second base wall; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole. a tubular duct, which is delimited by a tubular side wall and has an exhaust gas inlet opening, an exhaust gas outlet opening opposite the inlet opening, and an injection opening, which is obtained through the tubular side wall between the inlet opening and the outlet opening and is designed to receive an injector for a reducing substance; and a mixing body, which is arranged inside the tubular duct and creates a forced path to be followed by the exhaust gases in order to flow from the inlet opening to the outlet opening; wherein the mixing body comprises a first base wall with a partially circular shape, which faces the outlet opening of the tubular duct and partially engages the cross section of the tubular duct leaving free a passage area; wherein the mixing body comprises a second base wall with a circular shape, which faces the inlet opening of the tubular duct, completely engages the cross section of the tubular duct and has, at the centre, a main hole, which is coaxial to the tubular duct and is a through hole in order to allow for the passage of the exhaust gases; and wherein the mixing body comprises a first deflector wall, which extends from the first base wall towards the second base wall; the treatment device is characterized in that: the first deflector wall forms an acute angle with the first base wall and with the second base wall; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole. the first deflector wall forms an acute angle with the first base wall and with the second base wall; the first base wall partially engages the cross section of the tubular duct so as to leave free only one single passage area shaped like a half-moon and not containing any obstructions; and the second base wall has, at the centre, a single and one and only main hole.