An adjustable valve for a musical instrument, such as a trumpet, allows valve alignment to be adjusted without removing the valve from the instrument. The adjustable valve fits and looks like a standard valve. Rotation of a finger button of the valve causes an upper portion of an adjustable stem to rotate with respect to a lower portion of the adjustable stem that is threadably coupled to the upper portion, resulting in control over the length of the adjustable stem. Increasing or decreasing the length of the adjustable stem changes the alignment of the valve piston within the valve tube. Control over piston alignment is achieved by rotating the finger button, and thus is achieved without the need to remove the valve from the instrument.