<p>Proline, traditionally recognized as an osmoprotectant, has emerged as a multifunctional metabolite intricately involved in plant signaling and developmental regulation. Beyond its classical role in osmotic adjustment, proline modulates redox homeostasis, reactive oxygen species (ROS) detoxification, and metabolic reprogramming under stress conditions. Recent studies reveal its function as a signaling molecule that integrates stress perception with phytohormonal pathways such as abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and auxin. Proline also influences key developmental processes including root and shoot morphogenesis, floral induction, pollen fertility, embryogenesis, and seed maturation by coordinating redox cues and gene regulation. Furthermore, its participation in systemic acquired resistance (SAR) and legume–rhizobia symbiosis underscores its role in plant immunity and microbe interactions. Advances in understanding proline transport, mitochondrial catabolism, and gene regulation highlight its centrality in linking metabolic and signaling networks. This review consolidates current insights into the expanding roles of proline in plant signaling and development, emphasizing its potential as a metabolic hub for improving stress resilience and growth optimization in crops.</p>