The present application provides an armrest rotating mechanism suitable for being installed on an armrest tube and a frame. The armrest tube is rotatably connected to the frame. The armrest rotating mechanism includes an engagement device and an operation device. The engagement device is disposed between the armrest tube and the frame. Via the engagement device, the armrest tube can be positioned relative to the frame. The operation device is disposed on the armrest tube and connected with the engagement device. When the operation device is rotated, the engagement device is driven by the rotated operation device so as to disengage the armrest tube from the frame. Therefore, the armrest tube is adjustable relative to the frame and has a simple structure and utility convenience.