A new marine sponge, Antho ( Acarnia ) seogwipoensis n. sp., of the family Microcionidae, was collected from Seogwipo-si, Jeju-do, Korea, about 100 m in depth using a gill net on 1969. The genus Antho Gray, 1867 including Demospongiae, Poecilosclerida, Microcionidae, is a large group of sponges. About 100 species in Antho were reported from worldwide. The genus Antho contains five subgenera: Antho , Acarnia , Isopenectya , Jia , and Plocamia . Among them, about 30 species in Acarnia were described in world sponge. A new sponge’s body shape is branching, size up to 124 mm wide, 213 mm high, 3-8 mm thick in branch and 7-9 mm thick in stalk. Antho ( Acarnia ) seogwipoensis n. sp. is similar to A . ( A .) novizelanicum Ridley and Duncan, 1881 based on their spicules type and skeletal structure, but differs in the spicules dimension and growth form. This new species is branched growth form and have three kinds of toxa. The genus Antho Gray, 1867 (Demospongiae, Poecilosclerida, Microcionidae) is a large group of sponges. About 100 species in Antho were reported from worldwide ( Van Soest et al, 2014 ). The genus Antho contains five subgenera: Antho , Acarnia , Isopenectya , Jia , and Plocamia . Among them, about 30 species in Acarnia were described in world sponge fauna ( Van Soest et al., 2014 ). Five species in genus Antho and two in subgenus Acarnia have been reported from Korean waters ( Rho and Sim, 1972 ; Sim and Kim, 1994 ; Sim and Lee, 1998 ). The genus Antho is defined by having a choanosomal skeleton modified to a basal or axial renieroid reticulation of acanthose or occasionally smooth styles and/or strongyles ( Hooper and Van Soest, 2002 ). The subgenus Acarnia is characterized by acanthotylostrongyles forming the renieroid skeleton, less often acanthostyles, and a special category of echinating acathostyles overlap the main skeleton ( Hooper and Van Soest, 2002 ). A new marine sponge was collected from Seogwipo-si, Jeju-do, Korea in 100 m in deepth using a gill net on 12 December 1969. Specimens were fixed in 95% or 99.9% ethanol. Spicules were observed by light microscopy (Carl Zeiss Axio Imager A2; Germany) and by scanning electron microscopy (SEM, HITACHI S-3000N; Japan). Identifications were made on the basis of external features, shape, skeleton structure, and size and form of spicules. Thin free-hand sections were made with specimens shardened in alcohol using a surgical blade in order to observe the structure of skeleton. Spicules were prepared by dissolving a piece of sponge in sodium hypochlorite and examined with SEM ( Rützler, 1978 ; Hooper, 1996 ). The holotype is deposited in the Natural History Museum, Hannam University (HUNHM), Daejeon, Korea. Phylum Porifera Grant, 1836 Class Demospongiae Sollas, 1885 Order Poecilosclerida Topsent, 1928 Suborder Microcionina Hajdu, Van Soest and Hooper, 1994 Family Microcionidae Carter, 1875 Subfamily Ophlitaspongiinae De Laubenfels, 1936 Genus Antho Gray, 1867 1* Antho (Acarnia) seogwipoensis n. sp. (Figs. 1, 2) Type specimen. Holotype (por. 120), Korea, Jeju-do, Seogwipo-si, 12 Dec 1969, from 100 m in deep by gill net, Rho BJ, deposited in the HUNHM. Description. Body shape branching, size up to 124 mm wide, 213 mm high, 3-8 mm thick in branch and 7-9 mm thick in stalk. Texture, hard and well broken. Oscules 0.1-0.3 mm in diameter, scattered on surface. Colour unknown in life which gradually changeds to dark brown in ethanol. Surface rough and hispid with spicules. Ectosomal skeletal structure is plumo-reticulate. Choanosomal skeletal structure irregular reticulations composed of acanthostrongyles. Spongin fibres poorly developed. Spicules, megascleres, thick styles, size 200-(415)-630×15-(22.5)-30 μm and slender styles, size 180-(340)-500×3-(4)-5 μm with spines on tip of head. Acanthostrongyles, size 130-(140)-150×11-(13.5)-16 μm with spines both end. Microscleres, one category of palmate isochelae, size 15-(17.5)-20 μm. Three kinds of toxas, large toxas, size 120-(185)-250×2-(3.5)-5 μm, middle toxas, size 70-(80)-90×1-(1.25)-1.5 μm, small toxas, size 30-(45)-60×0.5-(0.75)-1 μm do not have spines on their ends. Etymology. This species named after the type locality, Seogwipo-si, Jeju-do, Korea. Remarks. Antho ( Acarnia ) seogwipoensis n. sp. is similar to A . ( A .) novizelanicum Ridley and Duncan, 1881 based on their spicules type and skeletal structure, but differs in the spicules dimension and growth form. This new species is branching growth form and have three kinds of toxa ( Hooper, 1996 ) ( Table 1 ).