Dietary fiber enrichment may modulate intestinal microbiota and positively impact metabolic dysfunction-associated steatotic liver disease (MASLD). This randomized, double-blind, placebo-controlled dual-center study evaluated the effects of dietary fiber (oat bran and spelt bran) on MASLD. After a 3-week Run-in phase during which dietary intake was assessed, 48 patients (CAP >280 dB, no fibrosis) were assigned to oat bran (4.5 g oat β-glucan, total fiber 11.7 g/day), spelt bran (11.7 g fiber/day), or placebo (2.1 g fiber/day) for 12 weeks. During the Run-in phase, dietary assessment alone significantly decreased BMI and liver enzymes (ALT, AST, γ-GT) while increasing microbiota diversity. Improvements were maintained in all three intervention groups. However, no significant changes were observed in hepatic steatosis (CAP), overall microbiota composition, and serum bile acid profiles. Dietary assessment alone improved MASLD biomarkers, with the fiber supplementation offering no additional benefit. This highlights the importance of dietary counseling in MASLD management. (clinical trials: NCT03897218).