This study investigated the dynamic changes and bioaccessibility of free and bound phenolics in rice bean (Vigna umbellata) during simulated gastrointestinal digestion. A total of 34 phenolic compounds were identified and quantified across oral, gastric, and intestinal phases by UPLC-MS/MS detected. Gastric digestion was identified as the critical stage for phenolic release, with multiple flavonoids increasing 2–3-fold, including rutin (205%), isoquercitrin (226%), and procyanidin B1 (134%). In contrast, in the intestinal phase, flavonoids including procyanidin B1, epicatechin, and quercetin became undetectable after extensive degradation, while phenolic acids such as p-hydroxybenzoic acid (157%) and trans-cinnamic acid (200%) accumulated gradually. Phloroglucinol showed a progressive accumulation increased continuously during digestion (10 to 36 mg/kg DW). Most bound phenolics remained remarkably stable, with over 85% retained throughout upper gastrointestinal transit, except for bound p-coumaric acid and phloroglucinol, which were gradually released. Notably, 3,4-dihydroxyphenylacetic acid was detected only in the bound form across all phases. These findings reveal the dual fates of rice bean phenolics, especially the bound fraction, and underscore the importance of their release and transformation during digestion when evaluating the bioactivity of rice bean polyphenols.