Previous research has often focused on studying the CNS damage in neuromyelitis optica spectrum disorders (NMOSD), while the role of the peripheral blood in the development of NMOSD is also of significant importance. The relationship between metabolites in blood and cerebrospinal fluid (CSF) with neuroimmune is receiving increasing attention. L-carnitine, whose astrocytic accumulation is associated with neuroinflammation and demyelination, may participate in the pathogenesis of NMOSD. However, whether circulating L-carnitine level has a causal effect on NMOSD risk needs elucidation. With large data sets now available, we used two-sample Mendelian randomization (MR) to determine whether circulating L-carnitine level is causally associated with the risk of NMOSD. Genetic variants associated with circulating L-carnitine levels were derived from a genome-wide association study (GWAS) of 7797 individuals from TwinsUK and KORA F4 cohorts. NMOSD summary statistics, including 215 cases and 1244 controls, were obtained from a separate GWAS. Subgroup analyses included aquaporin-4 (AQP4)-IgG-seropositive NMOSD (132 cases) and AQP4-IgG-seronegative NMOSD (83 cases). We used two-sample MR to explore associations between circulating L-carnitine levels and NMOSD risk, as well as its seropositive and seronegative subtypes. 16 SNPs (single nucleotide polymorphisms) were significantly associated with circulating L-carnitine level (P < 5 × 10), all of which were independent and available in the NMOSD dataset, after 1 SNP removed for being palindromic with intermediate allele frequencies in harmonization. Finally, a high circulating L-carnitine level conferred a protective effect against combined NMOSD (OR = 2.216 × 10, 95% confidence interval [CI] = 2.335 × 10-2.104 × 10, P = 0.0161) as well as AQP4-IgG-seronegative NMOSD (OR = 7.678 × 10, 95% CI = 2.233 × 10-2.640 × 10, P = 0.0082). There is no causal effect on AQP4-IgG-seropositive NMOSD risk (OR = 5.471 × 10, CI = 1.090 × 10-27.465, P = 0.2798) by circulating L-carnitine. Results remained positive and robust after the horizontal pleiotropy test, heterogeneity test, and Bonferroni test. In the reverse MR analysis, there was no causal effect of NMOSD and its subtypes on circulating L-carnitine levels. Our findings suggest that higher circulating L-carnitine levels may reduce the risk of NMOSD, particularly in AQP4-IgG-seronegative patients. L-carnitine could serve as a valuable biomarker and potential therapeutic target for NMOSD, especially in cases without AQP4-IgG. The genetic evidence from this study supports further exploration of L-carnitine's role in managing NMOSD.