2-Cyclopenten-1-one (2CP), which is a cyclic enone, has been considered an important precursor because of its versatile functionality in the synthesis of natural products and materials for biofuels. Here, we report the adiabatic ionization energy (AIE) and cationic structure of 2CP in the ionic transition between the neutral S0 and the cationic D0 states probed by high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the 0-0 band position in the VUV-MATI spectrum supported by the VUV-photoionization efficiency curve, the AIE of 2CP was determined to be 9.3477 ± 0.0004 eV (75,395 ± 3 cm-1), which is in good agreement with the reference value but much more accurate. The measured MATI spectrum combined with the Franck-Condon fitting at the B3LYP/cc-pVTZ level revealed that the cationic structure of 2CP is twisted with the C1 symmetry, whereas the neutral 2CP has the CS symmetry. The results indicate that geometrical changes induced by ionization are mainly attributed to the electron removal from the highest occupied molecular orbital, which consists of nonbonding orbitals on the oxygen atom in the carbonyl group interacting with the σ orbitals in the molecular plane of 2CP. Consequently, lowering the C1 symmetry for cationic 2CP led to the promotions of the ring-bending and ring-twisting modes in the MATI spectrum, which correspond to the ring puckering and CC twisting in the S0 state, respectively.