This is a Phase III, global, randomised, parallel-group, double-blind, multicentre study to assess the efficacy and safety of AZD6234 as an adjunct to incretin-based therapies, compared with placebo in adults with obesity or overweight with at least one weight-related comorbidity and with or without Type 2 Diabetes Mellitus (T2DM). Eligible participants will be randomised to either receive AZD6234 or matching placebo respectively. Study treatment (AZD6234 or placebo) will be added on to the participant's stable dose of background incretin-based therapies.