We determined the surface structure of RuO2(100) formed on Ru(1010) by employing the techniques of low-energy electron diffraction (LEED) and density functional theory (DFT) calculations. The RuO2(100) film grows lattice-matched with its [010] direction along the [0001] direction of Ru(1010) (4.5% compressively strained), while it grows incommensurately with the [001] direction of RuO2(100) along the [1.210] direction of Ru(1010). The RuO2(100) surface is terminated by bridging O atoms, which are attached to the coordinatively unsaturated Ru (cus-Ru) atoms with a bond length pf 2.01 Angstrom. The other Ru-O bond lengths are in the range of 1.90-2.05 Angstrom, typical for bulk RuO2. Due to the presence of cus-Ru atoms on RuO2(100), CO molecules adsorb quite strongly as evidenced by a desorption state at 300-400 K. The activity of the RuO2(100) surface for the CO oxidation reaction is similar to that of RuO2(110). The surface energies of RuO2(110) and RuO2(100) are 71 and 87 meV/Angstrom (2), respectively.