A high-gain and low-noise folded down-conversion mixer for <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>-band applications is presented in this paper. Benefited from the folded double-balanced architecture, the transconductance (<inline-formula> <tex-math notation="LaTeX">$g_{\mathrm {m}}$ </tex-math></inline-formula>) stage and the switch stage of the mixer can operate in-different bias conditions, providing a great freedom to optimize the two stages for noise reduction. By exploiting current-reuse and cross-coupled techniques, the conversion gain (CG) and noise figure (NF) of the mixer can be significantly improved. The proposed mixer has been designed and fabricated for verification in a 130-nm RF CMOS. Measured over the RF bandwidth from 23 to 25 GHz, the mixer achieves a maximum CG of 26.1 dB and a minimum NF of 7.7 dB under a local oscillator (LO) power of −3 dBm. The input 1-dB compression point (<inline-formula> <tex-math notation="LaTeX">$P_{\mathrm {1dB}}$ </tex-math></inline-formula>) is −17.8 dBm at RF frequency of 24 GHz. From 23 to 25 GHz, the LO-to-RF, LO-to-IF, and RF-to-LO isolations are better than 58 dB, 51 dB, and 43 dB, respectively. With a 1.5-V supply voltage, the mixer consumes an overall dc power of 16.8 mW. These measurement results clearly demonstrate that the proposed mixer has potential to be used in highly-integrated <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>-band CMOS radios.