Intermolecular proton transfer (IMPT) in a C-H···O hydrogen bonded dimer of an α,β-unsaturated aldehyde, methacrolein (MC), upon nonresonant multiphoton ionization by 532 nm laser pulses (10 ns), has been investigated using time-of-flight (TOF) mass spectrometry under supersonic cooling condition. The mass peaks corresponding to both the protonated molecular ion [(MC)H+] and intact dimer cation [(MC)2]•+ show up in the mass spectra, and the peak intensity of the former increases proportionately with the latter with betterment of the jet cooling conditions. The observations indicate that [(MC)2]•+ is the likely precursor of (MC)H+ and, on the basis of electronic structure calculations, IMPT in the dimer cation has been shown to be the key reaction for formation of the latter. Laser power dependences of ion yields indicate that at this wavelength the dimer is photoionized by means of 4-photon absorption process, and the total 4-photon energy is nearly the same as the predicted vertical ionization energy of the dimer. Electronic structure calculations reveal that the optimized structures of [(MC)2]•+ correspond to a proton transferred configuration wherein the aldehydic hydrogen is completely shifted to the carbonyl oxygen of the neighboring moiety. Potential energy scans along the C-H···O coordinate also show that the IMPT in [(MC)2]•+ is a barrierless process.