A tri-spine horseshoe crab, Tachypleus tridentatus ( Leach, 1819 ) was collected from a sandy beach of Udo Island of Jeju during the full moon tides in November 1997. Three spines on the posterior margin of the opisthosoma were characterized indicating that the specimen was T. tridentatus . This specimen was a female with the total length of 49.5 cm and prosoma width of 23.0 cm. Tachypleus tridentatus was the first record from Korean waters. A brief description and figures of the species were given. This finding indicates that Korea is included to the distribution range of the species. Horseshoe crabs have been known as “living fossils” because their fossil forms date as far back to 500 million years ago in the Paleozoic ( Sekiguchi, 1988 ). There are four species (one American and three Asian species) in the world: Limulus polyphemus ( Linnaeus, 1758 ) from North America to Mexico ( Shuster, 1982 ), Carcinoscorpius rotundicauda ( Latreille, 1802 ) from India to Philippines, Tachypleus gigas ( Müller, 1785 ) from India to Malaysia, and Tachypleus tridentatus ( Leach, 1819 ) from Malaysia to Japan ( Sekiguchi and Nakamura, 1980 ). They have been collected for food in Hong Kong, Thailand, and Malaysia ( Christianus and Saad, 2007 ; Shin et al., 2009 ) or for medical research ( Nakamura et al., 1986 ). Tri-spine horseshoe crab, T. tridentatus , is currently classified as a red list of threatened species or data deficient ( International Union for Conservation of Nature, 2014 ). Barnes (1974) showed in his book the range of this species along the Asian coasts from Japan and South Korea to the Philippines and East India. However, no references documenting the finding of this species from Korean waters have been found, nor were there any records of horseshoe crab specimens from Korea available in the collections ( The Korean Society of Systematic Zoology, 1997 ). During a visit to the Folklore and Natural History Museum in Jeju Special Self-governing Province, a specimen of T. tridentatus was found from the arthropod collections of the museum in 1997, which was collected from the sandy beach of Udo Island of Jeju in Korea. The specimen was described here as the first record of the species from Korean waters. The classification follows that of World Register of Marine Species ( WoRMS, 2014 ). Order Xiphosurida R. & E. Richter, 1924 Family Limulidae Leach, 1819 1* Genus Tachypleus Leach, 1819 2* Tachypleus tridentatus ( Leach, 1819 ) ( Figs. 1, 2A, B ) Limulus tridentatus Leach, 1819 : 537. Limulus longispina Van der Hoeven, 1838 : 32, pl. v. Tachypleus tridentatus : Pocock, 1902 : 263, pls. V, VI, fig. 2E; Okada et al., 1981 : 332; Nishimura 1995 ,: 8, pl. 73-1. Material examined . 1♀ (total length 49.5 cm, prosoma length 14.3 cm and width 23.0 cm, telson length 25.1 cm, middle part of telson width 7.5 mm and height 8.0 mm), Udo Island (Jeju), 17 Nov 1997, Yang KC, on coarse sandy beach, 33°30′18.457′′N, 126°56′29.808′′E. Description . Dorsal surface of prosoma smooth, convex, with posterior lateral angles prolonged backward approximately 1/2 length of opisthosoma; small median eye located anterior to each side of median ridge, compound eye located laterally to each lateral ridge ( Figs. 1A , B , 2A ). Prosoma concave ventrally; anterior dorsal surface reflected ventrally, forming triangular apex; 7 pairs of appendages present ( Figs. 1C , 3A ); first pair 3-segmented chelicerae ( Figs. 1C , 3A , B ); second pair pedipalps with chelate tips ( Figs. 1C , 3A , C ); third to sixth pairs 4 walking legs with chelate tips; last walking leg with movable spur distally on segment 4, with flat and strongly compressed segment 5, which longer than segment 6, with 4 leaf-like processes for pushing and sweeping away mud and silt during burrowing; seventh pair chilaria for food manipulation ( Figs. 1C , 3A , 4A ). Opisthosoma unsegmented dorsally, hexagonal, with 6 small pits along 2 median furrows; lateral margin with 3 movable spines ( Fig. 1A ): first spine slender and straight, second spine longest, third spine shortest and curved slightly ( Figs. 1A , C , 4B ); 3 small spines present on posterior margin ( Figs. 1A , 2A ). Opisthosoma with 6 pairs of appendages ventrally; first pair biramous genital operculum covering gonophores, inner ramous not reaching to tip of distal segment of outer ramous; last 5 pairs of appendages modified as biramous flap-like appendage (book gills) ( Figs. 1C , 4C ). Telson long, triangulate in cross section of middle part, crested above, slightly concave below ( Fig. 1 ) Color . Prosoma, opisthosoma, and telson glossy greenish brown. Lateral spines of opisthosoma pale brown. Eyes black. Habitat . This species hibernates on sandy to muddy bottoms in a depth of approximately 20 m in winter and migrates to the spawning beaches at night in the spring or summer ( Sekiguchi, 1988 ; Nishimura, 1995 ). Distribution . Malaysia, southwestern part of the Philippines, Taiwan, western part of Japan ( Sekiguchi and Nakamura, 1980 ), and now Udo Island of Korea ( Fig. 2B ). Remarks . Three Asian horseshoe crabs ( Carcinoscorpius rotundicauda , Tachypleus gigas , and T. tridentatus ) can be easily distinguished by the morphology of the cross section in the telson. It is triangular in two Tachypleus species (vs. circular in Carcinoscorpius ) ( Lazarus et al., 1990 ). In the triangular telson the width is subequal to the height in T. tridentatus (vs. shorter than the height in Tachypleus gigas ) ( Shuster, 2014 ). Also, based on the key of Pocock (1902) , this present specimen can be identified as T. tridentatus by having the posteroior margin of opisthosoma armed with three spines dorsally ( Fig. 2A ). It is female, with a pedipalp terminating with a chelate tip ( Fig. 3C ) and an opisthosoma bearing three movable spines on the lateral margin ( Figs. 1 , 4B ), whereas in male, the former is not chelate and the latter has six spines instead of three. Although the specimen described here (prosoma width 23.0 cm) is slightly smaller than the females (mean prosoma width 27.6 cm) reported from Japan by Wada et al. (2010) , it seems to be an adult.