A system for recirculating blow-by gases into an air intake duct of an internal combustion engine, includes a separating device to separate oil, in a flow of blow-by gases of the engine. The separator device has an inlet for receiving blow-by gases, at least one discharge for returning the oil separated to the engine, and an outlet for clean blow-by gases. A recirculation duct connects the outlet to an air intake duct of the engine. The recirculation duct has an end connected to the intake duct by a T-connector member, with a main duct portion arranged along the intake duct and a tubular element protruding inside the main duct portion. The tubular element protruding inside the main duct portion has an annular end surface that has an irregular profile, preferably in the form of a saw-like toothed profile.