A bethylid wasp is described as Neoapenesia makiharai n. sp. based on adults obtained from dead wood collected from the Ryukyus, Japan. The male is distinguishable from N. leytensis Terayama, the monotypic species of the genus, on the basis of the following characteristics: presence of short 2Rs vein, rounded propodeum, absence of sculpture on propodeum, widened subgenital plate, and apically rounded aedeagus. The female of Neoapenesia is described for the first time. Its morphological characteristics closely resemble those of Apenesia, but sex association was confirmed by the following facts: partial mitochondrial COI gene sequences (582 bp) of males and females were identical, and mating behavior was often observed. The female is easily distinguishable from females of Apenesia on the basis of the following characteristics: distinctly short antenna (not reaching posterior margin of head) and a disproportionately large head (head width about twice the dorsal pronotal width). Although females were active at night, males were active in the daytime and copulation was observed during this period. Since the female is larger than the male, phoretic copulation may not occur. Dead wood was collected and kept in plastic bags, from which a total of 16 species of coleopteran wood-borers and two predators were obtained together with N. makiharai n. sp. Many individuals of N. makiharai n. sp. were reared from dead wood severely affected by Cerecium longicorne and it was the most common species obtained from our wood samples. Therefore, the cerambycid species is considered a potential host species.