Background: Obesity is increasingly implicated in the development of multiple sclerosis (MS), but its effect on disease disability is less well-established. This study aims to investigate the association between obesity and MS severity utilizing Mendelian Randomization (MR). Methods: Employing a two-sample MR setting, we examined the effects of various obesity measures and adiposity distribution metrics on MS severity. Genetic proxies for body mass index (BMI) were selected from a study of 806,834 participants, with MS severity determined from a genetic study of age-related MS severity scores in 12,584 individuals with MS. Results: The main analysis reveals an association between elevated BMI and increased MS severity (P = 0.03). This is supported by a significant effect of whole body fat (P = 0.04), aligning with the hypothesis that obesity exacerbates MS disability. Sensitivity analyses suggest minimal heterogeneity and bias, indicating a potential causal effect. Conclusions: Our findings suggest that obesity adversely influences long-term disability outcomes in MS. The convergence of this genetic evidence with some of the prior observational studies strengthens the argument for a causal relationship between obesity and MS severity. These insights highlight obesity as a potentially modifiable risk factor in managing MS, underscoring the importance of weight management in MS treatment strategies.