Wheel hub motor (1) for an electric bicycle, comprising: a fixed shaft (2) which can be associated with a frame of an electric bicycle; a hub shell (3) which is rotatable with respect to said fixed shaft (3) and can be incorporated in a wheel structure (100) of said electric bicycle; an electric motor (4) being housed inside said hub shell (3), said motor having an internal stator (40) rigidly connected to the fixed shaft (2) and an external rotor (41) rotatably movable with respect thereto, and a reduction unit (5) which kinematically connects the rotation of said external rotor (41) to that of said hub shell (3) and which comprises two reduction stages arranged alongside each other in the axial direction inside the hub shell (3): a first reduction stage (50) comprising at least one first pinion (51), rigidly connected to the external rotor (41) and coaxial with the fixed shaft (2) and meshing with at least one first gear (52) rigidly connected to an eccentric shaft (53) mounted on an intermediate member; and a second reduction stage (55) comprising at least one second pinion (56) rigidly connected to said eccentric shaft (53) and meshing with a second gear (57) rigidly connected to said hub shell (3).