Device (11) for the automatic inserting of printed products or sheets (12) in individual envelopes (13), where the individual printed products or sheets (12) are fed by one or more feeders and the envelopes (13) are arranged piled up in a storage container (16) equipped with at least one gripping element (17, 18, 19) for feeding an individual envelope (13) between a first upper conveyor belt (20) and a first lower conveyor belt (21) facing each other to define a feeding group of the envelope (13) towards an inserting of products into envelopes zone, also foreseeing a second upper conveyor belt (15) and a second lower conveyor belt (14) facing each other to advance the printed products or sheets (12) one after the other, a nozzle (22) being associated with the first conveyor belts (20, 21) for opening and flipping back a tab (24) of the envelope (13) taken out from the storage container (16), the second conveyor belt (14) foreseeing an end (37) that can move forwards and backwards with respect to a sucked conveyor belt (35) aligned downstream of the conveyor belt (14) to define an opening (38) of variable size, at the opening (38) being foreseen a gripping element (39) of the tab (24) and at an initial portion of the sucked conveyor belt (35) being foreseen a gripping element (40) of the mouth of the envelope (13) locked onto the sucked conveyor belt (35) and introducing elements (45) that can be inserted over the tab (24) and inside the mouth of the envelope.