The integrity of the skin barrier is essential for maintaining cutaneous homeostasis, and its disruption contributes to various dermatological disorders. Postbiotics, defined as non-viable microbial products with established safety and stability, have attracted growing attention for improving skin health; however, the active components and mechanisms responsible for their barrier-repairing effects remain unclear. This study investigated the postbiotic of Limosilactobacillus reuteri CCFM1403 to identify its key bioactive substance and elucidate its role in skin barrier restoration. Using sodium dodecyl sulfate (SDS)-induced HaCaT cell damage and 1-fluoro-2,4-dinitrobenzene (DNFB)-induced murine barrier injury models, the L. reuteri CCFM1403 postbiotic significantly improved cell viability (restoring viability from 51.73% to over 80%), upregulated structural and functional barrier proteins, reduced transepidermal water loss (TEWL) to 14.55 g/m²/h, and enhanced skin hydration to 26.67%. Among the isolated fractions, the cell wall exhibited the strongest repair activity, and analysis identified peptidoglycan as the principal active compound. These findings demonstrate that cell wall–derived peptidoglycan is the primary bioactive component mediating the skin barrier–repairing effect of L. reuteri CCFM1403, providing mechanistic insight for the development of postbiotic-based functional food.