Self-locking handheld adhesive dispensers and methods for dispensing an adhesive using a handheld dispenser. The self-locking handheld dispenser includes a valve, a trigger connected to the valve, and a spring-biased trigger lock mechanism used to lock and unlock the trigger. A locking member of the trigger lock mechanism normally has a blocking relationship with the trigger that prevents an incidental force applied to the trigger from opening the valve. The trigger is unlocked by manually moving a locking member of the trigger lock mechanism out of the blocking relationship with the trigger. The trigger is then freed to respond to an applied manual force and open the valve to dispense the adhesive. When the trigger is released, the valve closes and the spring biasing of the trigger lock mechanism automatically returns the locking member to the blocking relationship with the trigger so that the locked state is restored.