ABSTRACT Objective This study aimed to investigate the associations of body roundness index (BRI) with cardiometabolic disease (CMD), cardiometabolic multimorbidity (CMM), and all‐cause mortality, while evaluating its impact across different stages of CMM progression. Methods In this prospective cohort study, 87,902 participants from the Kailuan cohort were categorized into BRI quartiles. Cox models estimated hazard ratios (HRs) and 95% CIs for the first occurrence of cardiometabolic disease (FCMD), CMM, and mortality. Multistate models assessed BRI's role across CMM progression. Results Over a median follow‐up of 13.68 years, 21,636 participants developed FCMD, 2114 developed CMM, and 14,782 died. Elevated BRI increased risks of FCMD, CMM, and mortality in Cox models. Multistate analysis revealed differential BRI effects across CMM progression: participants in the highest versus lowest BRI quartile showed HRs of 2.08 (1.99–2.17) for healthy‐to‐FCMD transition, 1.61 (1.38–1.88) for FCMD‐to‐CMM transition, and 1.09 (1.03–1.16), 0.99 (0.89–1.10), and 0.73 (0.54–0.99) for mortality from the healthy state, FCMD, and CMM, respectively. BRI's impact varied by disease type (diabetes mellitus, myocardial infarction, stroke) and sex, with stronger associations in females. Conclusions Our findings emphasize dynamic BRI monitoring as a biomarker for early CMM risk identification and prognostic assessment, necessitating disease‐ and sex‐specific prevention strategies.