An uncharacteristic prong of a buckle cast in bronze was found in 1983 during a rescue excavation of a multi-phase cemetery of the Lusatian, Przeworsk and Wielbark Cultures at Siemiątkowo, Żuromin County (Fig. 1). One part of the prong has the form of a decorative plate, with two recessed sides and a central opening (Fig. 2:1.2). It continues into the prong proper – slender, triangular of section, with a bowed terminal. The unusual form of the prong identifies it as a fragment of a buckle, type 8 group B, in the typology of R. Madyda-Legutko (1987, p. 13–15, 213, pl. 5). Finds of these buckles are extremely rare. To date, they have been recorded in three sites of the Luboszyce Culture and in two sites of the Wielbark Culture: Niedergurig, Distr. Bautzen in Saxony, Marxdorf, Distr. Märkisch-Oderland in Brandenburg, Grabice, Krosno Odrzańskie County, Ulkowy, Gdańsk County, and Gródek nad Bugiem, Hrubieszów County (Fig. 2:3–5). However, none of these finds has an openwork prong. The only type B8 specimen with a similar prong comes from the locality Wyszka, Pisz County (Fig. 2:6), which may suggest Balt roots of the buckle from Siemiątkowo. Similar decorative motifs may be found on openwork elements of the so-called Samland belts (Fig. 2:7.8). This suggests that the buckle fragment from Siemiątkowo could be one more example of Balt influence on the area of northern Mazowsze. Among the cited analogies only the artefacts from the Wielbark Culture cemeteries have a closer dating. In both assemblages of this culture the buckles were found together with brooches type Almgren 168, characteristic for the younger phase of the Late Roman Period. Thus, it seems justified to date the buckle prong from Siemiątkowo in the same way, and to link it with the Wielbark Culture which by the Late Roman Period had moved into areas of Mazowsze.