A passive micromixer with wavy channel walls with a sinusoidal variation is proposed, and numerical simulations based on Navier–Stokes equations are carried out for Reynolds numbers (Re) from 0.1≤Re≤30. Ethanol and water are used as the working fluids for mixing. A mixing index is employed to evaluate the performance of the micromixer. A different realization of chaotic mixing based on Dean vortices is observed with a shift in the center of rotation toward the inner sinusoidal wall for lower-wavelength mixers. The mixing index is found to sensitively increase as the wavelength of the sinusoidal channel walls decreases over the entire Re range considered. A parametric study is also carried out with the amplitude, offset-y, and number of cycles as the geometrical parameters. Specifically, Re=1 and 30 are chosen to demarcate the effects of these parameters in terms of the mixing performance for diffusion- (Re≤1) and recirculation- (Re≥10) dominated regimes. The amplitude turns out to be an important parameter that significantly affects the mixing performance. The proposed sinusoidal micromixer shows much better mixing performance than square-wave and zigzag micromixers for the same wavelength. The proposed micromixer can easily be realized and integrated with microfluidic systems such as lab-on-a-chip and micro-total analysis systems (µ-TAS) because of its simple in-plane structure.