We propose a compact, ultrabroadband and temperature-insensitive adiabatic directional coupler based on rib silicon waveguide-enabling arbitrary splitting ratios. Simulation results show that the device can achieve arbitrary splitting ratios from 1400 to 1600 nm, equal to 50%:50%, 60%:40%, 70%:30%, 80%:20%, and 90%:10% for the fundamental transverse electric mode. The designed device has an excess loss of less than 0.19 dB on the operational waveband. Furthermore, the proposed device shows a great robustness to fabrication imperfection, with a waveguide width deviation of 50 nm and ambient temperature change from 0°C to 200°C. These properties make the design a potential candidate for ultrahigh-density photonic integration chips.