The invention relates to a device for post-treating exhaust gases of a combustion engine (1), said device being characterized in that it comprises, in the engine exhaust gas flow direction of the engine: an oxidation catalyst (3); a mouth (41) of a means (4) for adding reducer or reducer precursor for the selective catalytic reduction of nitrogen oxides; a mixer (5) for mixing the exhaust gases and the reducer and/or converting the precursor into a reducer; and a catalyst for the selective catalytic reduction of the nitrogen oxides (6). Said device is characterized in that the above-mentioned elements are grouped together in a single casing (2). The invention also relates to an assembly comprising an exhaust collector and to a post-treatment device according to the invention. The invention moreover relates to a vehicle having such an assembly.