Background: Golimumab (GLM) is approved for the treatment of moderate to severe ulcerative colitis (UC). Higher serum concentrations of anti-tumor necrosis factor (TNF) agents are associated with improved clinical and endoscopic outcomes. Correlations between GLM serum concentrations and clinical and endoscopic outcomes were investigated during induction and maintenance treatment. In addition, a population pharmacokinetic model was developed to identify factors associated with the pharmacokinetics of GLM in UC. Methods: A prospective observational trial (GO-KINETIC) was conducted in patients with moderate to severe UC receiving induction and maintenance treatment with GLM. Clinical and endoscopic outcomes were evaluated, fecal GLM concentrations were measured and pharmacokinetic data were analyzed. Results: A total of 20 patients were enrolled. At week 8 (after induction treatment), 12 out of 20 patients (60%) showed an endoscopic response (≥1 point reduction in endoscopic Mayo score). Patients with endoscopic response at week 8 had numerically higher median GLM serum concentrations at week 2 compared to endoscopic non-responders: 9.1 µg/ml [5.9–12.3] vs. 7.1 µg/mL [5.2–9.0]; p =.384, respectively. At week 52, 3/20 patients (15%) achieved endoscopic remission (endoscopic Mayo score ≤1) and continued GLM treatment. Population pharmacokinetic analysis showed an inverse association between albumin concentrations and GLM clearance. GLM concentrations were undetectable in fecal samples. Conclusions: After induction therapy, 60% of the patients showed endoscopic response. During maintenance therapy, about one third of patients discontinued GLM treatment because of loss of response. These patients might benefit from dose optimization.