The rotational spectra of (RbO)-Rb-85 in the ground-and the first excited vibrational states and of (RbO)-Rb-87 in the ground vibrational state were observed in the millimeter- and submillimeter-wave region. The RbO radical was generated in a high-temperature absorption cell by the reaction of N2O with Rb vapor, which was produced in the cell by heating a mixture of RbCl and lithium metal to 500-550 degrees C. Analysis of the observed spectra yielded the rotational and centrifugal distortion constants, spin-rotation interaction constants, and hyperfine interaction constants. The bond length and the vibrational frequency were calculated from the rotational and centrifugal distortion constants to be 2.254 193 1 (15) Angstrom and 387.22 (20) cm(-1), respectively, for (RbO)-Rb-85. The ground electronic state of RbO was confirmed to be (2)Sigma, and the low-lying (2)Pi state was estimated to be located at 650-700 cm(-1) above the ground state by using a second-order perturbation expression with the vibrational energy in the denominator for the spin-rotation interaction constant. The observed hyperfine interaction constants indicated that the RbO radical is essentially an ionic molecule.