A security lock comprising a latch positioned at the edge of a door which is actuated to selectively lock or unlock the door. The security lock includes an optional key-actuated dead bolt which can provide additional security when needed. A mechanism for locking or unlocking the latch comprises a hand-actuated knob positioned on the inside of the door so that when the dead bolt is not in use, the latch mechanism may still be locked or unlocked simply by rotating the knob between two positions. The mechanism for locking the latch is disengaged when the latch is in the locked position. If the latch is in the locked position when the door is opened, when the door is closed the latch is automatically unlocked to prevent a person from accidentally being locked out.