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Chronic rhinosinusitis (CRS) is a highly prevalent and debilitating inflammatory condition of the upper airway, affecting 5-28% of the global population and imposing a substantial socioeconomic burden. Despite major advances in endoscopic sinus surgery, pharmacological management, and targeted biologic therapies, long-term disease recurrence following treatment remains an unresolved clinical challenge. Current pathophysiological frameworks centered on adaptive type 2 immunity, eosinophilic inflammation, and pathogen persistence fail to fully account for the chronification and therapy resistance of CRS. Emerging evidence positions trained immunity (the epigenetic and metabolic reprogramming of innate immune cells enabling non-antigen-specific functional memory) as a fundamental and previously underappreciated mechanism driving CRS recurrence. Persistent sinonasal microbial colonizers, including Staphylococcus aureus biofilms and fungal components, along with viral pathogens and dysbiotic microbiome communities, function as potent epigenetic training stimuli that reprogram sinonasal macrophages, group 2 innate lymphoid cells (ILC2s), and epithelial progenitor cells. The recent identification of a TLR4+ trained ILC2 subset in nasal polyp tissue, sustained by AP-1-driven chromatin remodeling at the Tlr4 locus, exemplifies the cellular specificity of this phenomenon. Concurrently, nasal basal stem cells acquire heritable pro-inflammatory chromatin states following type 2 cytokine exposure, encoding an epithelial inflammatory memory that perpetuates mucosal dysfunction independent of ongoing stimulation. This review systematically examines the microbial triggers, epigenetic mechanisms, key cellular mediators, and therapeutic implications of trained immunity in CRS, proposing a new framework for disease-modifying strategies targeting the sinonasal epigenetic inflammatory landscape.