The rotational spectra of cyclopentanone and its van der Waals complexes with argon and neon have been observed with a Balle-Flygare type pulsed jet Fourier transform microwave spectrometer in the 6 to 20 GHz region. This work improves the rotational constants and quartic centrifugal distortion constants for cyclopentanone and its five C-13 and the O-18 isotopologues. The argon-(C5H8O)-C-12-O-16 van der Waals complex has rotational constants of A = 2611.6688, B = 1112.30298, and C = 971.31969 MHz. The Ne-20-(C5H8O)-C-12-O-16 complex has rotational constants of A = 2728.8120, B = 1736.5882, and C = 1440.4681 MHz. In addition, the five unique, singly substituted C-13 and O-18 isotopologues of the argon complex are reported. The five single-substituted C-13 of the Ne-20 complex and the Ne-22-(C5H8O)-C-12-O-16 complex are reported. The rare gases are in van der Waals contact with the carbonyl alpha carbon and nearly in contact with the hydrogen on beta and gamma carbons toward the back of the ring.