Fruit ripening involves coordinated metabolic and molecular changes that shape quality traits, yet tissue-specific regulation in Selenicereus megalanthus remains unclear. Fruit development in S. megalanthus was investigated through an integrated analysis of pulp and peel across five developmental stages to elucidate tissue-specific metabolic and molecular regulatory dynamics. Proteomic profiling combined with targeted metabolomic analyses of sugars and phenolic compounds, multivariate statistics, and protein–protein interaction analysis was applied. A total of 411 proteins were identified in the pulp and 812 in the peel, of which 255 and 362 proteins, respectively, showed significant differential accumulation across development (p < 0.05), indicating higher regulatory plasticity in the peel. Multivariate analyses revealed clear stage-dependent reorganization of the proteome in both tissues. Functional annotation highlighted coordinated modulation of pathways related to cell wall remodeling, carbohydrate metabolism, antioxidant and detoxification systems, protein folding, and myo-inositol biosynthesis, directly associated with fruit texture and quality attributes. Metabolomic analyses revealed progressive sugar accumulation during ripening, with sucrose predominating at advanced stages, and pronounced tissue- and stage-dependent modulation of phenolic compounds, characterized by early enrichment in the pulp and sustained accumulation in the peel. Overall, these results demonstrate that yellow pitaya development involves tightly coordinated biochemical and regulatory mechanisms and identify the peel as a metabolically active tissue with relevance for postharvest management, fruit quality, and sustainable horticultural valorization.