Described is a unit for filling containers (2) forming single-use capsules (3) with a dose (33) of product for extraction or infusion beverages, comprising: a line (4) for transport of the containers (2); a station (SR) for filling the containers (2) with a dose (33) of product and comprising: a first containing seat (S1) designed to receive a dose (33) of product; a device (10) for moving the first seat (S); a device (11) for adjusting the position of the first containing seat (S1) between a position (P1) for receiving the dose and a position (P2) for releasing the dose; a substation (ST1) for forming the dose (33) inside the first containing seat (S1); a substation (ST3) for releasing the dose (33) of product from the first containing seat (S1) to a container (2) transported by the transport line (4), the adjusting device (11) being configured to place the first containing seat (S1) in the receiving position (P1) at the substation (ST1) for forming the dose (33) and in the release position (P2) at the substation (ST3) for releasing the dose (33).