Background <p>Findings on the association between body mass index (BMI) and risk of multiple myeloma (MM) are conflicting. Although some meta-analysis summarized available findings in this regard, some limitations, such as missing eligible articles and inclusion of ineligible articles, may have distorted their findings. Therefore, the current study was conducted to summarize available findings on the association between BMI and MM.</p> Methods <p>We searched online databases to identify prospective cohort studies that examined the association between BMI and risk of MM or its mortality.</p> Results <p>In total, 28 prospective cohort studies were included. During the follow-up periods ranging from 8.5 to 38 years, 18,923 cases of MM and 3,926 deaths from MM were recorded. The comparison between obese and normal-weight individuals revealed the summary relative risk (RR) of 1.17 (95% confidence interval (CI): 1.10–1.23) for MM incidence and 1.38 (95% CI: 1.25–1.52) for its mortality, indicating a significant positive association. Similar positive associations were also observed for overweight. Additionally, in the dose-response meta-analysis, each 5&#xa0;kg/m² increase in BMI was associated with a 2% higher risk of MM (RR: 1.02, 95%CI: 1.01–1.03) and a 3% higher risk of MM mortality (RR: 1.03, 95% CI: 1.02–1.05). However, we found no significant association between underweight and the risk of MM (RR: 0.90, 95% CI: 0.67–1.21) and its mortality (RR: 0.94, 95% CI: 0.71–1.24).</p> Conclusions <p>In conclusion, we found that high BMI, defined as obesity and overweight, was associated with an increased risk of MM and its mortality. PROSPERO ID: CRD42023467626.</p> Graphical Abstract <p></p>