Background: Obesity is prevalent in the Military Health System (MHS), including in the active-duty component. Obesity is a risk factor for severe COVID-19 outcomes, but less is known about the relationship with influenza-like illness (ILI) and influenza risk and symptom severity. In this study, we aim to examine (1) whether higher body-mass index (BMI) is associated with greater risk of ILI and influenza infection, and (2) whether BMI is associated with ILI symptom severity among influenza-vaccinated MHS beneficiaries. Methods: We used data from the Pragmatic Assessment of Influenza Vaccine Effectiveness in the Department of Defense (PAIVED) study collected between 2018 and 2022. PAIVED study participants (adult MHS beneficiaries) were randomly assigned one of three FDA-licensed vaccine types (egg-based, cell-based, and recombinant) and surveyed each week about ILI symptoms during the influenza season. Participants reporting ILI symptoms underwent influenza PCR testing and completed a quantitative ILI symptom score survey (FLU-PRO). In this analysis, we included the 8,762 (56.8%) PAIVED study participants who provided BMI information and responded to at least 40% of the weekly ILI symptom surveys. We assessed the association between BMI category (healthy/underweight [BMI<25], overweight [BMI 25-30], and obese [BMI ≥ 30]) and incidence of ILI and influenza using negative binomial regression models. We further evaluated the association between BMI and self-reported FLU-PRO symptoms (including nose, throat, eye, systemic, respiratory, gastrointestinal, and sensory symptoms dichotomized as “little to none” [scores 0-1] and “some to severe” [scores 2-4]) with GEE logistic regression models. In our regression models, we adjusted for age, race/ethnicity, education, military status, DoD affiliation, season, vaccine type, smoking status, and Charlson Comorbidity Index. Findings: Among the 8,762 participants evaluated, the mean age was 39.0 years, 59.8% were male, 68.7% were active duty, and 23.9% were obese. 28.1% of participants had at least one ILI, and 1.3% had PCR-confirmed influenza. Using the healthy/underweight group as reference, we found that risk of ILI was higher in the obese group (incidence risk ration (IRR) 1.34; 95% CI 1.20, 1.49); and the duration of ILI was longer in the obese group (estimate 0.10 days longer; 95% CI 0.06, 0.14). Risk of lab-confirmed influenza infection was higher in the overweight and obese groups, although not statistically significant. Participants with obesity reported greater symptom severity in all FLU-PRO domains, specifically, throat (odds ratio (OR) 1.47; 95% CI 1.11, 1.95), systemic (OR 1.43; 95% CI 1.04, 1.98), respiratory (OR 1.62; 95% CI 1.17, 2.25), gastrointestinal (OR 1.72; 95% CI 1.00, 2.95) and overall symptoms (OR 3.66; 95% CI 1.63, 8.23). Discussion/conclusion: Higher BMI is associated with an increased risk for ILI and greater ILI symptom severity even among MHS beneficiaries vaccinated against influenza. Further research is needed to understand if obesity is also associated with an increased risk of infection and severity by specific ILI pathogens in addition to influenza, including respiratory syncytial virus, enteroviruses, rhinoviruses, and adenoviruses. Additional investigation into whether obesity-associated ILI severity is mediated by increased inflammation and/or viral load may help guide future therapeutic options in those with higher BMI.