Exogenous application of bio-based nanomaterials provides a targeted strategy to modulate plant physiological and biochemical responses. This review synthesizes recent advancements in the foliar application of nanocellulose (NC), in particular, cellulose nanocrystals (CNC) and cellulose nanofibers (CNF), to enhance plant fitness. CNC-formed films provide physical and biochemical barriers that increase plant drought and cold stress tolerance. Topically applied CNC reduce non-stomatal transpiration and serve as insulators, allowing the flowering buds to successfully survive chilling, avoid freezing, and maintain cell membrane integrity. Simultaneously, CNC- and CNF-formed coatings are porous enough not to block the natural gas exchange essential for plants. CNC trigger internal antioxidant defense systems, upregulating reactive oxygen species-scavenging enzymes and modulating molecular signaling cascades. NC foliar treatment suppresses the growth of pathogenic bacteria and fungi, interferes with their adhesion and plant tissue penetration, and prevents biofilm formation. Thus, topical NC application could be regarded as a multi-functional tool for precision crop management and protection.