We evaluated the efficacy of once-daily (QD) upadacitinib 45 mg, an oral, reversible Janus kinase inhibitor, on early symptomatic improvement for ulcerative colitis (UC). Post hoc analyses were performed on pooled data from 2 replicate, phase 3, multicenter induction trials, U-ACHIEVE Induction and U-ACCOMPLISH, to determine the earliest time point of efficacy onset. Diary entry data through 14 days from the first dose of placebo or upadacitinib 45 mg QD were analyzed for daily improvement in UC symptoms (stool frequency, rectal bleeding, abdominal pain, and bowel urgency). Changes in inflammatory markers, high-sensitivity C-reactive protein (hs-CRP), and fecal calprotectin (FCP) were assessed at week 2 and quality of life (QoL) at weeks 2 and 8. Regression analysis determined the association between changes in UC symptoms and the likelihood of achieving clinical remission/response per Adapted Mayo score at week 8. Overall, 988 patients (n = 328 placebo, n = 660 upadacitinib) were analyzed. Patients treated with upadacitinib demonstrated significant improvements vs placebo in all UC symptoms between days 1 and 3 and maintained through day 14. A >50% reduction from baseline in hs-CRP and FCP levels was achieved by 75.7% and 48.2% of patients, respectively (P < .001 vs placebo). Increased rates of clinical remission/response per Partial Mayo score from week 2 (26.9%/59.4% upadacitinib 45 mg QD vs 4.3%/22.3% placebo, P < .001) and significant improvements in QoL at weeks 2 and 8 were observed. Early improvement in stool frequency and bowel urgency by day 3 and reductions in hs-CRP and FCP by week 2 were significantly associated with clinical remission/response at week 8. Upadacitinib 45 mg QD provided rapid relief of UC symptoms from day 1. Clinicaltrials.gov: U-ACHIEVE Induction (NCT02819635) and U-ACCOMPLISH (NCT03653026). Display omitted