Abstract: The present invention is a unique semi-automatic mop wringer. The mop wringer comprises a hollow housing vertically movably supported on a supporting structure by a suspension mechanism. The housing is engaged with the supporting structure through a rack-and-gear mechanism, so that a downward motion of the housing can be transmitted into the rotation of crank members which in turn drive a wringer roller to squeeze the mop against a wringer plate mounted inside the housing. A user can use the present invention mop wringer to wring a mop by placing the mop inside the housing, holding the mop holder and applying a downward force. The downward motion of the housing causes the wringer roller to move in a circular path to squeeze the mop against the wringer plate. When the user withdraws the downward force, the housing moves back upwardly to allow the user to repeat the process.