Phenolic compounds are pivotal in food preservation due to their potent antioxidant properties. Among these, p-coumaric acid (p-CA), a hydroxycinnamic acid widely present in fruits, vegetables, and cereals, was selected for this study due to its natural abundance and the limited mechanistic understanding of its radical-scavenging behavior. In this study, we investigated the reaction mechanism and fragmentation pattern of p-CA with 2,2-diphenyl-1-picrylhydrazyl (DPPH•) using ultra-high-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry (UHPLC-ESI-Q-TOF-MS/MS) in negative ion mode. The analysis revealed the formation of three distinct products: a p-coumaric acid-p-coumaric acid dimer (m/z 325.0709, RT 1.91 min), a p-coumaric acid-DPPH radical adduct (m/z 556.1093, RT 8.83 min), and a third compound with structural features consistent with a dimer-radical (m/z 718.1403, RT 7.68 min), called radimerSB-type structure. These data provide the first direct experimental validation of such fragmentation pathways for p-CA derivatives. According to the reaction mechanism, the p-CA may provide antioxidant activity mainly by radical adduct formation (RAF). Although similar radical-derived adducts have been previously reported for ferulic acid, this is the first study to describe the formation of such a structure from p-CA, highlighting a novel antioxidant pathway and expanding the understanding of phenolic reactivity. MS/MS fragmentation confirmed the unique structural features of each product. Furthermore, the antioxidant activity of p-CA, as measured by DPPH radical scavenging and IC₅₀ determination, was comparable to that of the standard gallic acid and sinapic acid. These findings contribute to a deeper understanding of the antioxidant mechanisms of phenolic acids and support the relevance of p-CA as a functional ingredient in food systems aimed at oxidative stability.