A square-law harmonic-rejection mixer (SL-HRM) driven by a sine local oscillator (LO) is proposed for wideband and tunable rejection of all LO harmonics. The key performances including the gain, noise figure (NF), input-referred third-order intercept point (IIP3), and harmonic rejection ratio (HRR) with respect to the amplitude and purity of the sine-LO are analyzed. In the circuit level, the SL-HRM incorporates a voltage-boosted technique to surmount the gain and NF penalties of square-law mixing, while guaranteeing the device reliability via introducing a supply startup circuit and a node-voltage protection circuit. The entailed high-purity sine-LO is achieved via a digital-intensive square-to-sine LO generator plus a passive- RC low-pass filter. Fabricated in 65-nm CMOS, the VHF-/UHF-band SL-HRM has a tunable gain of 1-13 dB, associated with an NF of 27-15 dB, under an LO amplitude of 50-200 mV pp. The IIP3 is stable within from +6.5 to +7.6 dBm, as expected. The tunable HRR3 and HRR5 are 13-40 dB and 17-45 dB, respectively. The mixer core consumes 7.5 mW at 2.5 V, while the LO generator draws 3-6 mW at 1.2 V. The total active area is just 0.042 mm2.