A heterogeneously integrated widely tunable III-V-on-silicon ring laser with unidirectional operation is demonstrated. 40 nm tuning range (from 1560 nm to 1600 nm) is obtained using the Vernier effect between two ring resonators incorporated in the ring laser cavity. Unidirectional operation is obtained by integrating a DBR reflector coupling the clockwise and counterclockwise mode of the ring laser cavity. Unidirectional operation is obtained over the entire tuning range with about 10 dB suppression of the clockwise mode. The laser linewidth is lower than 1 MHz over the entire tuning range, down to 550 kHz in the optimum operation point. The waveguide-coupled output power is above 0 dBm over the entire tuning range.