A rotary drive device has a fluid-actuated rotary drive which has a drive housing and a drive unit rotatable about a main axis relative to the drive housing. The drive unit contains a pivot piston which divides two drive chambers from one another in the interior of the drive housing, which can be supplied with compressed air fluidic pressure medium controlled by a control fluid channel system, in order to cause a rotational movement of the drive unit. The rotary drive is equipped with a pressure detecting device, which enables pressure detection of the fluid pressure prevailing in the two drive chambers by means of pressure detecting channels formed separately with respect to the control fluid channel system. A robot arm is also proposed, which includes the rotary drive device as an arm joint.