Background: Although some studies have explored the relationships between dietary fiber (DF) supplement and gut barrier function, changes of gut microbiota, and clinical outcomes in critically ill patients, the results were not consistent. Objective:The purpose was to explore the effect of DF on gut barrier function, gut microbiota, short-chain fatty acids (SCFAs), inflammation, and clinical outcomes in critically ill patients. Methods:A search was performed through five databases from inception to July 12, 2021. Data were expressed as mean difference (MD) or odds ratio (OR) with CI.Results: Twenty-one studies involving 2084 critically ill patients were included.There was a significant reduction in intestinal permeability, demonstrated by lactulose/rhamnose ratio (MD, −0.04; 95% CI, −0.08 to −0.00; P = 0.03) on day 8, Creactive protein on day 14 (MD, −36.66; 95% CI, −44.40 to −28.93; P < 0.001) and duration of hospital stay (MD, −3.16; 95% CI, −5.82 to −0.49; P < 0.05) after DF supplement. There were no significant differences in SCFA levels, duration of mechanical ventilation, and mortality between two groups. However, subgroup analysis results indicated significant decreases in duration of hospital stay and risk of mortality were seen in the subgroups with a supplementary fiber dose ≥20 g/day (MD, −5.62 [95% CI, −8.04 to −3.21; P < 0.0001]; OR, 0.18 [95% CI, 0.06-0.57; P = 0.004]), as well as in the medical intensive care unit (MD, −4.77 [95% CI, −7.48 to −2.07; P < 0.01]; OR, 0.13 [95% CI, 0.03-0.65; P < 0.05]). Conclusions: DF may improve gut barrier function, modulate intestinal microbiota, decrease systemic inflammatory response, and advance clinical outcomes in critically ill patients.