Chronic rhinosinusitis (CRS), with or without nasal polyps (CRSwNP/CRSsNP), is a prevalent inflammatory condition of the sinonasal mucosa, for which intranasal corticosteroids (INCS) are widely used as first-line therapy. This systematic review evaluated the efficacy and safety of INCS, used alone or with adjunctive treatments, in improving clinical outcomes in CRS patients. A comprehensive literature search was conducted across PubMed, Cochrane Library, Scopus, Web of Science, and the Virtual Health Library (VHL) through May 2025, including randomized controlled trials (RCTs) assessing INCS efficacy in adults and children with CRS. Outcomes examined included nasal polyp score (NPS), nasal congestion, quality of life (QoL), sinus opacification, olfactory function, acute exacerbations, and adverse events. Six high-quality RCTs involving 2,339 participants were included. Dupilumab demonstrated the greatest NPS reduction (Δ-2.06), improved nasal congestion, and reduced the need for surgery or systemic steroids. Mepolizumab, omalizumab, and benralizumab were also effective in reducing polyp size and symptoms, particularly in eosinophilic CRS. EDS-FLU (exhalation delivery system with fluticasone) improved congestion, SNOT-22 (22-item Sino-Nasal Outcome Test) scores, and reduced acute exacerbations. In pediatric patients, mometasone delivered with saline nebulization significantly improved SN-5 scores. All interventions were well tolerated, with few adverse events reported. Overall, INCS are effective in managing CRS symptoms and reducing disease burden, particularly when combined with biologics or advanced delivery systems, and remain a cornerstone of CRS management with a strong safety and efficacy profile across diverse populations.