A reactor (1; 30) for cleaning fibrous suspensions dispersed in liquids, comprising: a substantially cylindrical container (2); one or more rotating blades (3) arranged inside the container (2); a plurality of feed units (5, 6, 7, 8, 9) for feeding the suspension to be cleaned into the container (2) together with a gaseous substance dispersed therein in the form of bubbles; means (14; 16) for intercepting the suspension arranged inside the container (2); means (10) for skimming the foam and the cleaned suspension. Each feed unit (6, 7, 8, 9) comprises at least one feed pipe (6a, 7a, 8a, 9a) which extends into the container (2) and is provided with a plurality of ejectors (6d, 7d, 8d, 9d), each of which have a spray direction (Z) facing the inner surface (2a) of the container (2) for the distribution of the suspension to be cleaned starting from the inside of the cross section (S) of the container (2).