The copper alloy rung brooch (Armbrustsprossenfibel) discovered in the spring of 2013 on a wooded sand dune in Radość housing estate in southern district of Warszawa-Wawer is a stray find lacking in context. No other traces of prehistoric occupation were identified in the immediate vicinity of its discovery (Fig. 1). The brooch has a pseudo-crossbow construction – its chord is a purely element. On the surface of the fibula is a design of engraved straight lines and zigzag, presumably made with a roulette, and stamped sub-triangular and circular motifs (Fig. 3). Surviving almost complete the brooch misses only a fragment of its pin. The decoration on the bow and on coils of beaded wire on spring terminals are substantially worn, presumably due to extended use. The brooch is a late variant of rung brooches attributable to group IV variant B of M. Rudnicki, dated to phase E2b and onset of phase E3, which corresponds to late 6th or the very onset of 7th century (M. Rudnicki 2008, p. 297, pl. 13; cf. J. Kowalski 2000, p. 223–224). Their form is similar to the one illustrated by N. Åberg (1919, fig. 182, p. 27). Rung brooches are characteristic for the territory inhabited by Balt tribes, in particular the area settled by Prussians, between the Baltic Sea, the Vistula and the Neman rivers (cf. J. Okulicz 1973, p. 5); their largest number is known from grave inventories of the Olsztyn Group, i.a., from Tumiany and Kielary, Olsztyn County (Fig. 4:1–5), and of the Elbląg Group, i.a., from Nowinka, Elbląg County (Fig. 4:6.7). Outside these concentrations rung brooches are recorded in Sambia and, much more rarely, in Lithuania (Fig. 5). Two exceptional specimens were discovered in Brandenburg: a fragmented brooch of an older type at Seetz, Kr. Perleberg (H. U. Voß 1991, fig. 2) and a younger specimen, similar in form to the brooch from Warszawa-Wawer, at Prützke, Lkr. Potsdam-Mittelmark (Fig. 5:6a.b; W. von Unverzagt, J. Hermann 1958, pl. 1:b; W. von Unverzagt 1960, fig. 1a–c). The connection of rung brooches to the territory Prussian settlement is sufficiently strong to treat this form as ethnically diagnostic. Consequently, the brooch find from Wawer could document the presence of Prussians in Mazowsze at the time of emergence of a new tribal and demographic situation, i.e., in the period 6th–7th c. AD, between the decline of Roman Period settlement and the full emergence early Slav culture.