It has been reported that supplementing certain amino acids has therapeutic effects on ulcerative colitis (UC). We intend to explore whether citrulline (Cit) supplementation has protective effects on UC. Fifteen male Wistar rats were divided into normal control group (NC group), UC group and UC+Cit group, with five rats in each group. The UC model was established by TNBS/ethanol method. Rats in UC+Cit group were intragastrically administered with Cit for 7 consecutive days after modeling. All rats were sacrificed after 7 days. Blood samples were collected to detect the number of monocytes. Colon tissues were taken for HE staining. Immunohistochemistry staining for CD68 and p-STAT3 were performed to detect the infiltration of monocytes and the phosphorylation of STAT3 in colon tissues. The concentrations of MCP-1, IL-6 and IL-17A and the protein expression of p-STAT3 in colon tissues were measured by ELISA and western blot methods, respectively. The body weight of UC group rats decreased significantly after 7 days (p<0.05). However, the weight loss of UC+Cit group rats was not statistically significant (p>0.05). The number of peripheral blood monocytes in UC+Cit group was significantly lower than that in UC group (p<0.05), and the infiltration of CD68-positive monocytes in the colon tissue of UC+Cit group was significantly reduced than that in UC group. The concentrations of MCP-1, IL-6 and IL-17A and the expression of p-STAT3 in colon tissues of UC+Cit group rats were significantly lower than those in UC group (both p<0.05). Our study suggests that Cit supplementation may be a potential therapy for UC.