An injection and mixing device comprises a casing having an exhaust inlet and an exhaust outlet, the casing internally defining a passage for exhaust gases from the exhaust inlet to the exhaust outlet. The exhaust inlet opens into an inlet region of the passage. The injection and mixing device further includes an injector of a liquid comprising a nitrogen oxide reducing agent or a precursor of such a reducing agent. A deflector is housed within the casing and defines in the passage a first passage connecting the inlet region to the exhaust outlet, in which the exhaust gases flow at a first average velocity, and a second passage connecting the inlet region to the exhaust outlet, in which the exhaust gases flow at a second average velocity lower than the first average velocity, The injector injects liquid into the second passage.