The salticid spider genus Stertinius Simon 1890 currently contains twelve species that are distributed in Southeast Asia and Japan (WSC 2020). Of these, only S. kumadai Logunov, Ikeda & Ono 1997 is currently known from Japan.Upon examining spider materials obtained from the Ryukyu Islands, Japan, I recognized an undescribed member of this genus. This species is described as new in this paper. In addition, both sexes of S. kumadai are redescribed here, since the male of this species has been figured only in a Japanese illustrated book without detailed description (Ono, Ikeda & Kono 2009).Specimens were preserved in 80% ethanol, and their morphological features were observed under a stereomicroscope, Leica M125C. Type series are deposited in the collection of the Department of Zoology, National Museum of Nature and Science, Tokyo.The following abbreviations are used: ALE, anterior lateral eye; AME, anterior median eye; PLE, posterior lateral eye; PME, posterior median eye; RTA, retrolateral tibial apophysis.All measurements are given in millimeters. Leg measurements are given in the following format: [femur + patella + tibia + metatarsus + tarsus = total]. Genus Stertinius Simon 1890 [Japanese name: Minamitsuya-haetorigumo-zoku] Stertinius Simon 1890, p. 135 (type species: Stertinius dentichelis Simon 1890, from Guam Is., Mariana Isls.). Remarks. As noted by Logunov et al. (1997) and Prószyński & Deeleman-Reinhold ( 2013), the genus is still somewhat poorly defined as the type species S. dentichelis is not well known.