In this study, a highly integrated wideband double-balanced up-conversion mixer with digital step-variable gain for dual-frequency systems is proposed. Here, a 4-bit inductorless T-type passive attenuator is employed to showcase the attainment of a 15 dB gain control range (GCR) with 1 dB increments, and phase fluctuation is minimized by using capacitive compensation. Active baluns are employed at all intermediate frequency (IF), local oscillator (LO), and radio frequency (RF) ports to reduce size and achieve wideband performance for both IF and LO. Furthermore, two transmission paths with high isolation are incorporated for dual-frequency systems. Fabricated in a 40 nm complementary metal-oxide-semiconductor process, the chip occupies a total area of 2.67 mm² and consumes 91.5 mW of direct current power. With an LO power level of −7 dBm, the proposed up-mixer demonstrates a step-variable gain ranging from 10 to 24.2 dB and from −4 to 10.2 dB in the two paths. Additionally, the high-gain path exhibits a measured 3-dB frequency bandwidth of 1.1–3.35 GHz with an input 1-dB compression point (IP_1dB) of −13 dBm, while the low-gain path shows a measured 3-dB frequency bandwidth of 0.7–3.05 GHz with an IP_1dB of −2 dBm. The 3-dB IF bandwidth ranges from 0.01 to 0.8 GHz, and the measured root-mean-square (rms) amplitude errors are below 0.25 dB for both paths.