The invention disclosed is a weir valve with an adjustable bypass incorporated into the weir portion of the valve housing. The valve housing has an interior chamber with an inlet portion, a middle portion and outlet portion. An elongate weir traverses the middle portion of the interior chamber and separates the inlet portion and outlet portion. The top surface of the weir defines a valve seat. A diaphragm traverses the interior chamber and is moveable by a compressor onto the weir effecting a closure of the valve. The adjustable bypass consists of a bypass chamber which extends longitudinally of the elongate weir with inlet and outlet openings centrally located on the sides of the weir between the bypass chamber and the inlet portion and outlet portion respectively. A bypass valve member is adjustably moveable in the bypass chamber to variably restrict the communication between the inlet and outlet openings.