The rate of weight reduction during obesity treatment declines over time and eventually reaches a weight plateau. We investigated factors associated with time to weight plateau (TTWP) in tirzepatide-treated participants with obesity or overweight in a post-hoc analysis of SURMOUNT-1 and SURMOUNT-4 trials. Participants adherent to tirzepatide treatment and achieving ≥5% weight loss by primary endpoint (week 72 SURMOUNT-1; week 88 SURMOUNT-4) were included. Weight plateau was defined as a weight change <5% over a 12-week interval and all subsequent 12-week intervals. TTWP was time from randomization to the start of the first 12-week interval. Association between baseline characteristics and TTWP was assessed. Overall, 1438 participants in SURMOUNT-1 and 259 in SURMOUNT-4 were included. Across BMI categories (overweight, class I, II, and III), median TTWP in SURMOUNT-1 was 24.3, 26.0, 36.1, and 36.1 weeks, respectively (p <.05, class II and III vs. overweight). By week 72, 90.2%, 88.9%, 87.6%, and 87.8% of participants in SURMOUNT-1 had reached a weight plateau across respective BMI categories [Correction added on 22 January 2025, after first online publication: The "72%" has been changed to "72" in this version.]. Higher doses of tirzepatide (10/15 mg), younger age, and female sex were more likely to reach a weight plateau later. Results in SURMOUNT-4 were similar. In this post-hoc analysis, most participants reached a weight plateau by week 72. Higher doses of tirzepatide, younger age, and female sex were associated with a longer TTWP. Further research into modifiers of weight reduction phases with tirzepatide may inform treatment decisions for its use in chronic weight management. Clinical Trial Registration: ClinicalTrials.gov, identifiers NCT04184622 (SURMOUNT-1) and NCT04660643 (SURMOUNT-4), available at http://www.clinicaltrials.gov/.