Production of lumpfish (Cyclopterus lumpus) has become crucial in controlling sea lice levels in salmonid aquaculture. To improve their breeding, there is a need for early sex identification. The genomic region containing anti-Müllerian hormone (amh) gene was suggested as the candidate sex-determining gene in lumpfish. However, the genome of lumpfish contains three copies of amh with ambiguous sex specificity, designated amh1, amh2, and amh3. The study aims to analyse the male-specific region between these amh paralogues, for its application as a sex marker. In this study, we utilised polymerase chain reaction (PCR)-based assays to identify the male-specific amh markers in lumpfish and estimate the length of the male-specific region in the lumpfish genome. Our results suggest that a region of approximately 26 kilobase (kb) region containing amh1 and amh2 is male-specific, while both sexes share amh3. The developed PCR-based genetic sex identification assays targeting amh1 and amh2 exhibited more than 97% accuracy. Further experiments in other members of the Cyclopteridae: Aptocyclus ventricosus, Eumicrotremus taranetzi, and E. asperrimus revealed male-specific amh fragments only in A. ventricosus. Phylogenetic analyses using the available Cyclopteridae amh sequences suggest that male-specific amh arose early in the Cyclopteridae lineage. These findings and development of the PCR test will be of service to lumpfish aquaculture as well as to future studies attempting to further elucidate the sex-determination system and sex chromosome evolution in lumpfish.