Background: Obesity is a well-established risk factor for postmenopausal hormone-receptor positive breast cancer (BC). The role of obesity in premenopausal BC, especially in women under the age of 40, is not well understood. Multiple studies have shown a decreased rate of premenopausal BC with increasing body mass index (BMI), notably for estrogen receptor positive tumors. Even though there is an inverse association between obesity and premenopausal BC, it has been suggested that obesity has a negative prognostic role in premenopausal BC. This may be explained if premenopausal obese women develop more aggressive tumors. Obesity is associated with triple negative and Luminal B cancers in premenopausal women. We sought to evaluate the relationship between premenopausal breast cancer and obesity in a diverse patient population. Methods: We assembled a 10-year database of women diagnosed with breast cancer before age 40 in a single health system. Data collected included age at diagnosis, self-identified race, breast cancer subtype, BMI, and stage at diagnosis. The control group was all women under the age of 40 presenting to our health system in the same time period. Stratification analysis was performed using Mantel-Haenszel Risk Estimation. Results: The risk ratio between the standardized estimates of BC patients under 40 with BMI ≤ 30 to BMI > 30 is 0.7924, 95% CI (0.683 – 0.920), P = 0.0022. This implies that young BC patients are more likely to be obese than all young women presenting to our health system. There are more obese Black patients compared to White patients. The effect of obesity on BC diagnosis was more pronounced in Black patients, with a risk ratio of 0.59 compared to 0.96. Conclusions: The data presented here challenge previous research which had shown that obesity seemed to be protective against breast cancer in premenopausal women. Here, we see a statistically significant increase in obesity among young women < 40 years presenting with breast cancer. The increased risk of breast cancer diagnosis was particularly pronounced in obese Black women. Previous studies have had a very low proportion of Black women represented, which may cause underestimation of risk in this patient population. These data suggest that obesity is a risk factor for breast cancer in young women, especially in Black patients.