The main aim of the study is to assess the effect of RO7204239 in combination with tirzepatide, compared to placebo in combination with tirzepatide, on body weight loss after 48 weeks of treatment in adults with obesity or overweight with at least one weight-related comorbidity, but without diabetes mellitus (DM). The study comprises of a 4-week screening period; a 48-week core treatment period, where all participants will receive tirzepatide as background treatment and will be randomized to one of the 4 treatment arms; a 24-week treatment extension period, where participants will stop treatment with tirzepatide and a 24-week post-treatment follow-up (FU) period.