Leptin is a multifunctional protein involved in processes such as body weight regulation, energy expenditure, fat metabolism, food intake, and appetite regulation. Duplicate leptin genes, leptin-a and leptin-b, were previously detected in the orange-spotted grouper. In this study, we cloned the full-length open reading frame (ORF) of the leptin-a gene in the orange-spotted grouper, searched for polymorphisms, and performed association analyses between these polymorphisms and seven growth traits. Six polymorphisms, consisting of 2 SNPs in intron 1 (c.182T > G, c.183G > T) and 4 SNPs in exon 2 (c.339C > G, c.345C > T, c.447G > A, c.531C > T), were identified and genotyped in 200 individuals. The c.182T > G and c.183G > T polymorphisms showed complete linkage and were analyzed together. Association analyses revealed that the c.182 + 183TG > GT polymorphism was significantly associated with body weight (BWT) and body width (BWH), with the AB (TG/GT) genotype showing positive effects on growth traits. Additionally, the SNP c.447G > A was significantly associated with BWT, BWH, overall length (OL), trunk width (TW), and head length (HL), with the GA genotype displaying positive effects on growth traits. The c.531C > T SNP showed a close association between the TT genotype and decreased growth. Our results demonstrate that several polymorphisms in the leptin-a gene are associated with growth traits and can be used for marker-assisted selection (MAS) in orange-spotted grouper populations.