Aim Most patients with ulcerative colitis (UC) with active mucosal disease have a lower C-reactive protein (CRP) level than the classic accepted cutoff level (<= 5 mg/l). We aimed to predict the mucosal remission in UC with an optimal cutoff level of CRP when mucosal activity and extensiveness of UC were both considered. Method In this retrospective study, we evaluated CRP values and their relation to mucosal extension and UC activity in 331 colonoscopic examinations performed between December 2016 and March 2019. Endoscopic activity and disease extension were assessed using Mayo scores and the Montreal classification. Results The Mayo 2 and 3 groups' CRP values were significantly higher when compared with Mayo 0-1 between values of E1 and both E2 and E3 with an increasing trend. The standard CRP cutoff level <= 5 mg/l only yielded 55% specificity in predicting mucosal remission. In the ROC analysis, a CRP cutoff level <= 2.9 mg/l predicted an overall mucosal remission (Mayo 0-1) with 77% sensitivity and 80% specificity, and <= 1.9 mg/l predicted Mayo-0 with 70% sensitivity and specificity. In the clinical remission subgroup, the overall CRP cutoff level was even lower, at <= 1.58 mg/l. Conclusion An overall CRP cutoff level <= 2.9 mg/l predicts mucosal remission in UC better than the standard cutoff <= 5 mg/l. Mucosal remission in stable clinical remission may present with an even lower CRP level. An increasing trend in the CRP level from E1 through E3 even in mucosal remission suggests that both histological inflammation and extensiveness may have some influence on a CRP-based prediction of endoscopic remission.