The present invention provides an improved adjustable chair headrest. Specifically, the present invention comprises a double articulating dental chair headrest apparatus which has a one-handed side pushbutton release and locking mechanism. When pressed, the pushbutton temporarily disengages a movable mating plate having splined perforations away from each of two corresponding rotatable splined pin assemblies to unlock each splined pin assembly. The unlocked splined pin assemblies each rotate independently. Each splined pin assembly is also fixedly attached to one of a headrest and a backrest via an attachment plate. The two splined pin assemblies are held in relation to each other, the movable mating plate having splined perforations, and the side pushbutton release and locking mechanism, by a housing component.