The genus Sphathius is belong to the subfamily Doryctinae in the family Braconidae and are described with >300 species in the world (Shenefelt and Marsh, 1976). This genus is distinguished from other winged Doryctinae by 1) forewings with three submarginal cells; 2) first subdiscal cell of forewing closed; and 3) first metasomal segment petiolate, widening apically, lacking basal wing-like projections, and inserted on propodeum near coxal bases (Marsh, 1997). For the purpose of finding the biological agent for Asian longhorned beetle (ALB; Anoplophora glabripennis (Motschulsky)) and Citrus longhorned beetle (CLB; A. chinensis (Förster)), we had installed the ALB- and CLB-infested sentinel logs to attract parasitoids at experimental field of the Korea National Arboretum during summer season in 2014. Several L1 and L2 CLB larvae were found attacked by braconid larvae that were reared to adult stage. Their larvae are identified to Spathius gwangneungensis sp. nov. belong to the Spathius exarator species group. This new species has special characteristics as following; Body reddish brown or dark reddish brown, body length 2.5~3.7mm and forewing length 2.2~3.0mm in female and body length 2.1~2.6mm and forewing length 1.5~2.0mm in male, hind femur distinctly transversely aciculate dorsally, forewing more or less infuscate with a longitudinal hyaline band medially, vein m-cu of hind wing interstitial, second tergite entirely sculptured without striation, and third to fifth tergites basally sculptured. Ovipositor sheath 0.6 times as long as metasoma.