Objective: Evaluate the association between body mass index (BMI, kg/m2) and one-year mortality among people who survived the first 90 days after spinal cord injury (SCI). Design: Cohort study. Setting: Eighteen SCI Model Systems centers throughout the United States. Participants: 6640 participants (men, 79.6%; mean age, 42.8 ± 17.7y; Whites, 62.3%) who had an SCI between October 2006 and March 2017. Interventions: Not applicable. Outcome Measures: All-cause mortality and causes of death. Life table method was used to estimate mortality rates, while Cox proportional hazard model was conducted to assess the impact of BMI on mortality after adjusting for demographic and injury-related factors. Results: Based on BMI obtained during initial rehabilitation, participants were classified into underweight (4.2%), normal weight (41.2%), overweight (30.9%) and obese (23.8%) groups, and their corresponding one-year mortality rates were 2.6%, 1.8%, 3.1%, 3.5%, respectively (P = 0.002). After adjusting for potential confounding factors, people with obesity had a higher mortality risk than those with normal weight (hazard ratio, 1.51; 95% confidence interval, 1.00–2.28). The most frequent causes of death for people with obesity were infective and parasitic diseases and respiratory diseases, while respiratory diseases were the most frequent for people with other BMI statuses. Conclusion: People with obesity who incur an SCI need special attention to prevent early mortality. Future studies should explore factors that contribute to such a higher mortality after SCI, such as preexisting conditions and comorbidities. The effects of BMI on long-term mortality also deserve further investigation.