This paper proposes an Internal Model Principle (IMP) based unified digital periodic controller (UDPC) for power converter systems. The proposed UDPC offers a unified framework for housing various periodic controllers with universal sufficient stability criteria for digital periodic control systems. Moreover, a set of rules-of-thumb for optimal gain tuning for digital periodic controllers are derived to assure the rapid overall convergence rate of the periodic errors. An application example of UDPC controlled single-phase PWM converters is given to confirm the effectiveness of the proposed UDPC control scheme.