Background/Objective: Ingestion of dietary fiber can influence in the remission of patients with ulcerative colitis (UC). There are no current recommendations for fiber intake in UC; therefore, we evaluate the association between dietary fiber and the activity of the disease. Methods: Ours is a cross-sectional study in patients with a confirmed diagnosis of UC to whom a 24 h recall was applied; this allowed for the estimation and classification of type of dietary fiber. The patients were divided into two groups: (1) remission and (2) active UC. We analyzed the quantity and type of fiber with the grades of disease activity through Spearman correlation and logistic regression. Results: A total of 152 patients were included; it was found that those with clinically active UC consumed less total fiber (p = 0.016) and insoluble fiber (p = 0.018). Meanwhile, in endoscopic grade, the difference was for insoluble fiber (p = 0.038). Insoluble fiber had an inversely significant correlation with fecal calprotectin levels (r = -0.204; p = 0.018). Logistic regression showed that less than 11 g of insoluble fiber was a risk factor for clinical activity (OR = 2.37; 95% CI 1.107-5.019; p = 0.026). Conclusions: Consumption below the current recommendation of total and insoluble dietary fiber is associated with clinical activity of UC.