A security lock (28) is described which permits a door (10) to be opened by a predetermined amount while maintaining security against forced entry. Preferably, the security lock (28) also functions as a deadbolt lock and comprises a slotted floor plate (36) covering an aperture (38) in the floor (20), a vertical bolt (34) extendable into the aperture (38), and a lock mechanism (30) which raises and lowers the bolt (34) into and out of engagement with the floor plate (36). In a first engaged position, the end (50) of the bolt (34) is locked into engagement with the floor plate (36) in such a manner that the door (10) may be partially opened. However, the bolt (34) cannot be removed from the floor plate (36) with the door open, thus providing security against forced entry through the partially open door. In a second engaged position, with the door (10) completely closed, the bolt (34) may be raised or lowered into or out of engagement with the floor plate (36), similar to a conventional deadbolt lock. The security lock (28) therefore provides improved security and convenience over known devices such as door chains and security doorstops.