A bicycle bottom bracket assembly has a shaft having a longitudinal axis X-X, a pair of bearings configured to support the shaft in rotation with respect to a housing box of the bottom bracket assembly provided in a bicycle frame, and a pair of adapter devices configured to be cantilevered with opposite free ends of the box to support the bearings outside of said box. At least one of the adapter devices has at least one first stop element and at least one second stop element arranged on opposite sides with respect to at least one bearing, wherein the first and second stop elements cooperate with the at least one bearing to keep the at least one bearing inside a space having a predetermined axial extension. The bearing's movement is thus limited in the axial direction, providing a fixed reference to determine the chain line of the bottom bracket assembly.