A rotor (3, 3', 3") for an aircraft (1) is described that has a mast (150), an attenuating device (7, 7', 7") to attenuate the transmission of vibrations from the mast (150) in a plane orthogonal to the first axis (A); and a transmission device (15) interposed between the mast (150) and the attenuating device (7, 7', 7"); the attenuating device (7, 7', 7") comprises a first and a second mass unit (8, 9) with a first and a second mass (10, 11) rotatable about the first axis (A) with a first and a second rotational speed ((N-1)*Ω; (N+1)*Ω); (N+1)*Ω; (N-1)*Ω)), two control units (40, 40') operable to cause an additional rotation of at least one of the first and second masses (10, 11); and a first and a second support assembly (41, 41'; 42, 42') carrying the first and second masses (10; 11); each control unit (40, 40') controls the angle between the first and second masses (10, 11) and each control unit (40, 40') comprises: a belt (50) coupled to the support assembly (41, 41'; 42, 42') and a drive unit (51, 51') coupled to the first belt (50), to cause the rotation of the first support assembly (41, 41'; 42, 42') with respect to said transmission device (15).