Prosigoj (Serbian: Просигој, Greek: Προσηγόης) was a Serbian ruler believed to have ruled prior to c. 830. Serbia was a Slavic principality subject to the Byzantine Empire, located in the western Balkans, bordering with Bulgaria in the east. Mentioned in the De Administrando Imperio (DAI) from the mid-10th century, he succeeded his father Radoslav and was succeeded by his son Vlastimir (r. c. 830–851).
The son of Radoslav, and grandson of Višeslav, the first Serbian ruler by name, Prosigoj is believed to have ruled some time before c. 830, or until 835. One of these most likely ruled during the revolt of Ljudevit of Lower Pannonia against the Franks (819–822). According to Einhard's Royal Frankish Annals, Ljudevit fled from his seat at Sisak to the Serbs (who were believed to have been somewhere in western Bosnia) in 822, with Einhard mentioning "the Serbs, who control the greater part of Dalmatia" (ad Sorabos, quae natio magnam Dalmatiae partem obtinere dicitur). At this time, there was still peace with Bulgaria. His son Vlastimir is the eponymous founder of the Vlastimirović dynasty, which ruled until c. 960.
The four named succeeding Serbian rulers are not mentioned in the Chronicle of the Priest of Duklja (CPD), a source dating to c. 1300–10 and largely discredited in historiography (the CPD is deemed useless for events in the Early Middle Ages). Instead, the CPD mentions several historically unconfirmed or legendary rulers, Svevlad, Selimir, Vladin and Ratimir, although it maintains the patrilineal succession tradition. According to Sima Lukin Lazić (1863–1904), Prosigoj was dead by the time of a Bulgar attack on Serbia following the Bulgar conquest of Frankish-held Banat and Syrmia.