Introduction: Functional gastrointestinal disorders, such as irritable bowel syndrome (IBS), significantly impair quality of life. The low–fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) diet has emerged as an effective dietary intervention. This systematic review evaluates the evidence regarding its efficacy and clinical applicability in adults with functional gastrointestinal disorders. Methodology: A systematic review was conducted in accordance with the PRISMA 2020 guidelines and registered in PROSPERO (CRD1005339). Studies published between 2015 and 2025 were included if they evaluated the clinical effectiveness of the low-FODMAP diet compared with a standard diet or control group in adults with functional gastrointestinal disorders. Randomized controlled trials, cohort studies, and systematic reviews with meta-analyses were considered. Study selection, data extraction, and risk-of-bias assessment were independently performed by two reviewers using the Cochrane RoB 2.0 tool. Results: Of 1,750 records identified, 37 full-text articles were reviewed, and 22 studies were included in the final analysis. Findings demonstrated a significant reduction in gastrointestinal symptoms, improvements in quality of life, and adequate dietary adherence. Conclusions: The low-FODMAP diet is effective for symptom management in adults with irritable bowel syndrome. Its clinical implementation should be individualized and accompanied by professional supervision in order to optimize therapeutic outcomes.