A tunable directional coupler for radio frequency (RF) front-ends of handheld mobile devices is presented. The coupler is implemented in a 0.13 μm bulk RF-CMOS technology with all elements monolithically integrated on a single die. A digitally-tuned termination network at the isolated port of the coupler is realised solely with RF-switch transistors using the channel resistance of a metal-oxide-semiconductor (MOS) device. The fine-tuned termination network allows the coupler to achieve a directivity of over 25 dB in the whole cellular frequency range from 699 MHz up to 2.7 GHz. The worst-case insertion loss in the transmitted path is 0.15 dB at 2.7 GHz.