The radial piston rotary device with compact gear drive mechanism includes a base, a casing mounted to the base, and a gearbox coupled to the casing. The casing houses a piston assembly with a pair of first radial pistons and a pair of second radial pistons coaxially mounted to the first radial pistons with each pair rotating at asynchronous, cyclically varying velocities. During a power cycle, a leading pair of radial pistons powers the associated gear assembly, and the gear assembly drives a flywheel assembly. A drive shaft axially extends from the flywheel assembly outside the gearbox to be selectively coupled to a driven component. The flywheel drives the pair of lagging radial pistons. During operation, each pair of radial pistons rotate at cyclically varying angular velocities out of phase with each other, yet produce a uniform output to the drive shaft.