A locking Y-valve has branches that can be independently locked open or locked closed. Each branch has a ball valve connected to a rotating valve column. The valve column extends through a lock plate and attaches to a housing that contains a locking mechanism. The locking mechanism has a retractable lock pin that cooperates with two recesses in the lock plate to keep the housing, valve column, and ball valve locked in place. The locking mechanism is attached to a rotatable handle outside the housing. Rotating the handle retracts the lock pin from a recess so that the handle can be moved. Releasing the handle extends the lock pin into a recess, securing the handle in place. One or more of the lock plate and housing may be integral with the Y-valve. In the preferred embodiment, the lock plate is configured to be retrofit onto existing Y-valves, so that a non-lockable Y-valve may be converted into a lockable Y-valve.