The spionid polychaete Dipolydora armata, a borer in calcareous substrata, is recorded for the first time from Belize, Brazil, Taiwan, and Vietnam. Specimens from these and other localities, as well as the type material of Polydora armata from Madeira Island and Polydora rogeri from the Mediterranean were examined and all the specimens were considered to be conspecific. Dipolydora armata is up to 8 mm long (usually 2–3 mm), with up to 45 segments (usually 25–35 segments), an incised prostomium, caruncle until the end of segment 2, up to 10 pairs of branchiae from segment 7, up to 20 awl-like modified spines per notopodium in up to 15 posterior segments, bilobed or cup-shaped pygidium, hooded hooks from segment 7 accompanied by capillaries throughout the body, major falcate spines of segment 5 with a large lateral tooth and an apical structure covered by fine bristles and appearing as a cowling or third tooth on the convex side of the main fang. The life history of the species includes a period of asexual reproduction by architomy beginning soon after settlement, then sexual maturation and continuous breeding within an extended period with production of lecithotrophic larvae developing entirely inside egg capsules. Once mature, individuals probably reproduce only sexually and do not undergo additional architomic divisions. Asexual reproduction results in high morphological variability of adult individuals, particularly in number, size, and arrangement of awl-like spines in notopodia. Polydora rogeri is placed into synonymy of D. armata. The original interpretation of the relationship between the polychaetes and the excavating sponge Cliona viridis as mutualistic symbiosis is discussed. Dipolydora armata is considered to be a widespread non-specialized borer perforating various calcareous substrata.