Oxo and amino substituted purines and pyrimidines have been suggested as protonucleobases participating in ancient pre-RNA forms. Considering electromagnetic radiation as a key environmental selection pressure in early Earth, the investigation of the photophysics of modified nucleobases is crucial to determine their viability as nucleobases’ ancestors and to understand the factors that rule the photostability of natural nucleobases. In this Letter, we combine femtosecond transient absorption spectroscopy and quantum mechanical simulations to reveal the photochemistry of 4-pyrimidinone, a close relative of uracil. Irradiation of 4-pyrimidinone with ultraviolet radiation populates the S1(pp*) state, which decays to the vibrationally-excited ground state in a few hundreds of femtoseconds. Analysis of the post-irradiated sample in water reveals the formation of a 6-hydroxy-5H-photohydrate and 3-(N-(iminomethyl)imino)propanoic acid as the primary photoproducts. 3-(N-(iminomethyl)imino)propanoic acid originates from the hydrolysis of an unstable ketene species generated from the C4-N3 photofragmentation of the pyrimidine core.