We examine the role of spin-orbit coupling in the electronic structure of , in which Ru ions in configuration form a honeycomb lattice. Our x-ray absorption spectroscopy measurements at the Ru edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be important for understanding magnetism of this compound.