Background <p>Brown adipose tissue (BAT) regulates metabolic homeostasis, yet its role in tumor presence remains undefined. This study investigates the associations among BAT, body mass index (BMI), and tumor presence in a large Chinese cohort.</p> Methods <p>We retrospectively analyzed 1,736 adults who underwent ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography (¹⁸F-FDG PET/CT) at Shanghai East Hospital (2016–2024). Patients were grouped by BAT status with propensity score matching and sex stratification. BAT was identified by FDG uptake at typical depots with CT density criteria. Statistical analyses included chi-square tests, regression models, and mediation analysis (structural equation modeling with bootstrapping) to assess the role of BMI in the BAT-tumor presence association.</p> Results <p>The presence of BAT was associated with younger age, female sex, and lower BMI. In multivariable logistic regression, age (<i>p</i> &lt; 0.001) and BMI (<i>p</i> = 0.014) were independently associated with tumor presence. After propensity score matching (PSM), sex-stratified mediation analysis showed that in females, BAT was indirectly associated with a lower likelihood of tumor presence (Natural Indirect Effect OR = 0.88, 95% CI 0.72–0.98), whereas no significant mediation effect was observed in males. Among cancer patients, individuals with BAT had a significantly lower BMI compared to individuals without BAT (overall: 21.08 vs. 23.34; females: 21.33 vs. 22.97; males: 19.65 vs. 24.68).</p> Conclusions <p>Our findings indicate that BAT and tumor presence are statistically associated in a sex-specific manner, with BMI potentially playing a mediating role in females. The consistently lower BMI among BAT(+) cancer patients further indicates a plausible link between BAT and body weight regulation. These observations warrant further investigation to elucidate the underlying mechanisms.</p>