Parasitism upon Cyclosa omonaga (Araneidae) by the ichneumonid ectoparasitoid Reclinervellus nielseni is reported for the first time, based on molecular identification, corroborating that R. nielseni is a specialist parasitoid of members of the genus Cyclosa, having previously been recorded from C. argenteoalba,C. conica and C. laticauda. The parasitoid larva at the time of discovery (11 December) was still tiny in spite of the beginning of winter, indicating it was in its overwintering phase, and this is the first case of the overwintering R. nielseni larva found in Japan. The host's cocoon web, with fibrous and fluffy thread decorations (FTD) resulting from the wasp's behavioural manipulation, produced by the parasitized C. omonaga is described and illustrated, and its potential role is discussed.