There are fewer therapeutic options for patients with active ulcerative colitis (UC) compared to patients with active Crohn's disease (CD) and the investigators are facing a persistent unmet need for additional effective and affordable therapies for patients with UC. Methotrexate (MTX) 25 mg once weekly administered subcutaneously (sq) or intramuscularly (im) is an efficient therapy to induce and maintain steroid free remission in patients with CD. To evaluate the efficacy of a similar approach in patients with active ulcerative colitis the investigators conduct a double-blind, placebo controlled, randomized, multicenter, parallel group trial to investigate the safety and efficacy of 25 mg MTX applied subcutaneously once weekly in patients with active UC, who either failed 5-ASA therapy, or are steroid dependent or are intolerant or not responding to azathioprine/6-mercaptopurine therapy or have no response/ lost response to infliximab prior to the study inclusion. The study is designed as a drug withdrawal trial and includes two periods, the Induction Period (week 0-16) and the Maintenance Period (week 17-48). In the open label Induction Period every patient will receive a steroid taper, MTX 25 mg sq once weekly + daily folic acid 1 mg tablets for the induction of clinical response or remission. Patients responding to the open label MTX therapy and being off steroids between week 12-16 will be randomized at week 16 1:1 to Placebo sq once weekly + daily folic acid 1 mg tablets + 2.4 g mesalamine or to MTX 25 mg sq once weekly + daily folic acid 1 mg tablets+ 2.4 g mesalamine. The Specific Aims of the trial are: i) To evaluate the safety and tolerability of 25 mg MTX applied sq once weekly over a time period of 48 weeks; ii) To evaluate the relapse-free survival of MTX maintenance therapy compared to placebo over a time period of 32 weeks; iii) To evaluate the efficacy of MTX over a time period of 16 weeks to induce steroid free remission; iiii) To establish a DNA, plasma and serum library to enable the evaluation of clinical and pharmacogenomic models to predict the response to MTX therapy in patients with UC. With 25-30 participating centers actively enrolling, the investigators anticipate to complete enrollment for this study in a time period of 3 years. Completion of this trial will define the therapeutic value of MTX in UC, potentially changing the current therapeutic strategy in UC.