An exhaust gas treatment device for an internal combustion engine; the exhaust gas treatment device comprises: a first tubular body with a cylindrical shape, which develops around a longitudinal symmetry axis and has a first inlet opening with a circular shape, which is perpendicular to the longitudinal axis, and a first outlet opening with a circular shape, which is parallel to and opposite the first inlet opening and perpendicular to the longitudinal axis; a first catalyst monolith, which is arranged inside the first tubular body and has a first inlet surface, through which the exhaust gases get into the first catalyst monolith; a second tubular body, which is directly fixed to the first tubular body and has a second inlet opening with a circular shape and a second outlet opening with a circular shape, which is parallel to and faces the first inlet opening of the first tubular body; a second catalyst monolith, which is arranged inside the second tubular body and has a second inlet surface, through which the exhaust gases get into the second catalyst monolith; a third tubular body, which is hollow on the inside, is directly fixed to the second tubular body and has a third inlet opening with a circular shape, which can be connected to an outlet of a turbine of a turbocharger of the internal combustion engine, and a third outlet opening with a circular shape, which is parallel to and faces the second inlet opening of the second tubular body; and the second inlet opening of the second tubular body is perpendicular to the second outlet opening of the second tubular body; the exhaust gas treatment device is characterized in that: the third inlet opening of the third tubular body forms, with the first inlet surface of the first catalyst monolith, a first obtuse angle (α) ranging from 105° to 135° and preferably equal to 115°; and the third inlet opening of the third tubular body forms, with the second inlet surface of the second catalyst monolith, a second acute angle (β) ranging from 17° to 27° and preferably equal to 22°. a first tubular body with a cylindrical shape, which develops around a longitudinal symmetry axis and has a first inlet opening with a circular shape, which is perpendicular to the longitudinal axis, and a first outlet opening with a circular shape, which is parallel to and opposite the first inlet opening and perpendicular to the longitudinal axis; a first catalyst monolith, which is arranged inside the first tubular body and has a first inlet surface, through which the exhaust gases get into the first catalyst monolith; a second tubular body, which is directly fixed to the first tubular body and has a second inlet opening with a circular shape and a second outlet opening with a circular shape, which is parallel to and faces the first inlet opening of the first tubular body; a second catalyst monolith, which is arranged inside the second tubular body and has a second inlet surface, through which the exhaust gases get into the second catalyst monolith; a third tubular body, which is hollow on the inside, is directly fixed to the second tubular body and has a third inlet opening with a circular shape, which can be connected to an outlet of a turbine of a turbocharger of the internal combustion engine, and a third outlet opening with a circular shape, which is parallel to and faces the second inlet opening of the second tubular body; and the second inlet opening of the second tubular body is perpendicular to the second outlet opening of the second tubular body; the exhaust gas treatment device is characterized in that: the third inlet opening of the third tubular body forms, with the first inlet surface of the first catalyst monolith, a first obtuse angle (α) ranging from 105° to 135° and preferably equal to 115°; and the third inlet opening of the third tubular body forms, with the second inlet surface of the second catalyst monolith, a second acute angle (β) ranging from 17° to 27° and preferably equal to 22°. the second inlet opening of the second tubular body is perpendicular to the second outlet opening of the second tubular body; the exhaust gas treatment device is characterized in that: the third inlet opening of the third tubular body forms, with the first inlet surface of the first catalyst monolith, a first obtuse angle (α) ranging from 105° to 135° and preferably equal to 115°; and the third inlet opening of the third tubular body forms, with the second inlet surface of the second catalyst monolith, a second acute angle (β) ranging from 17° to 27° and preferably equal to 22°.