Abstract: A novel and rapidly tunable laser was proposed, consisting of a semiconductor ring laser (SRL) formed by an active microring under constant current pumping and a monolithically integrated external tunable passive microring reflector. The geometry, therefore, enables very fast tuning by separating the mechanism (the active microring cavity) that defines stable lasing mode frequencies from the tuning mechanism (the passive microring) that only selects one of these stable frequencies. Furthermore, compared to the laser based on grating structures, this SRL offers many promising advantages including strong mode-selection and simple fabrication, which does not require phase matching sections and provides a strong immunity of the output wavelength to the thermal fluctuations of the tunable section. Theoretical simulations based on a multimode rate-equation model were used to study the performance of the laser. Numerical simulations demonstrate digital tuning of the laser across 15 cavity modes with reasonably low threshold current of about 40 mA and moderate side-mode suppression ratio (SMSR).