An infusion assembly, particularly for machines for preparing beverages, comprising a first shoulder and a second shoulder which face each other and are adapted to define internally a space for the movement of a piston, a dispensing assembly being arranged between the first and second shoulders, means for locking a capsule being provided, the piston being rigidly coupled to the locking means in order to move the locking means from an open position to a closed position in which the capsule is accommodated within the dispensing assembly.