The electronic transition spectrum of ruthenium monoxide (Ru0) molecule in the spectral region between 545 nm to 640 nm has been recorded and analyzed using laser ablation/reaction free-jet expansion and laser induced fluorescence spectroscopy. The RuO molecule was produced by reacting laser-ablated ruthenium atoms with N2O seeded in argon. Nine vibrational bands were recorded, and they are identified to belong to four electronic transition systems, namely, the [18.11S2 4-X5A4, [16.0]Omega = 5-X-5 Delta(4), [18.1]Omega = 3-X-5 Delta(3), and [15.8]Omega = 4-X-5 Delta(3) systems. RuO was determined to have a X-5 Delta(4) ground state. A least-squares fit of the measured rotational lines yielded molecular constants for the ground and the low-lying electronic states. A molecular orbital energy level diagram has been used to help with the assignment of the observed electronic states.