We report on a compact, broadband, 2×2 adiabatic 3 dB coupler using sub-wavelength gratings (SWGs) for silicon-on-insulator waveguides. In our device, two SWG waveguides that support two transverse electric modes and have tapered waveguide widths were used to achieve an adiabatic mode evolution of the two-waveguide system for broadband 3 dB power splitting. We present results for a SWG adiabatic 3 dB coupler that has an overall coupler length of 50 μm and achieves broadband power splitting over a 130 nm wavelength range with an imbalance of no greater than ±0.3 dB and with low excess losses of less than 0.5 dB.