The impact of body mass index (BMI) on survival in lymphoma remains controversial. We leveraged a prospective cohort of lymphoma patients enrolled to SPORE Molecular Epidemiology Resource between 2002 and 2015 to assess the association of BMI before diagnosis, BMI at diagnosis, and BMI change over time with lymphoma-specific survival (LSS). A total of 4009 lymphoma patients (670 diffuse large B-cell lymphoma (DLBCL), 689 follicular lymphoma (FL), 1018 chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), and 1632 other subtypes) were included. Significantly shorter LSS after diagnosis was observed for FL patients who were obese before diagnosis (HR: 3.02, 95%CI: 1.43-6.41, p=.004) and for those with a ≥ 5% increase in BMI from diagnosis to 3-year follow-up (HR: 3.53, 95%CI: 1.22-10.2, p=.020). In contrast, obesity prior to or at the time of diagnosis was not associated with LSS in DLBCL and CLL/SLL. The impact of weight control after diagnosis in FL patient warrants investigation.