A Viterbi decoder synchronization circuit comprises a phase shifter for introducing a variable amount of delay time to a received bit stream of convolutional codes in response to a control signal applied thereto with respect to a word synchronization signal which is derived from the bit stream. A first detector detects maximum and minimum metric values of the Viterbi decoder. A second detector detects the difference between the detected maximum and minimum metric values for coupling to an integrator. The output of the integrator is applied to a third detector which detects when the integrator output reaches a value indicative of a word-in-sync or word-out-of-sync condition. A phase shift signal is generated in response to an output signal from the third detector and applied to the phase shifter as the control signal.