A rotating device for an armrest is connected with a fixing mount disc on a top surface of an armrest seat and is rotatably coupled with a rotary plate on a bottom surface of the support plate. The fixing mount disc includes a conical groove and a threaded orifice formed on the conical groove, and the conical groove has a toothed slot. The rotary plate includes a post corresponding to the conical groove and the toothed slot, the rotary plate also includes an extending fringe extending outwardly from an outer side thereof to contact with a peripheral side of the fixing mount disc, wherein the post has two recesses, each recess extends outwardly from each of two sides of the post to accommodate each of two slidable retainers, and each slidable retainer is pushed by a spring and has a locking tab for retaining with the toothed slot.