Vlastimir (Serbian Cyrillic: Властимир, Greek: Βλαστίμηρος; c. 805 – 851) was the prince of Serbia from c. 830 until c. 851. Little is known of his reign. He held Serbia during the growing threat posed by the neighbouring, hitherto peaceful, First Bulgarian Empire, which had expanded significantly toward Serbia.

At the time, the Bulgarian Empire and the Byzantine Empire were at peace by treaty, and although the Byzantine Emperor was overlord of the Serb lands, he was unable to aid the Serbs in a potential war. Presian I of Bulgaria eventually invaded Serbia, resulting in a three-year-war, in which the Bulgarian army was devastated and driven out. Vlastimir then turned to the west, expanding well into the hinterland of Dalmatia. He is the eponymous founder of the Vlastimirović dynasty, the first Serbian dynasty.

The unnamed 7th-century Serbian ruler (prince, archon) who led the Serbs to the Balkans and received the protection of Heraclius (r. 610–641) was an ancestor of Vlastimir. The Serbs at that time were organized into župe, a confederation of village communities (roughly the equivalent of a county), headed by a local župan (a magistrate or governor). According to Fine, the governorship was hereditary, and the župan reported to the Serbian prince, whom they were obliged to aid in war. Emperor Constantine VII Porphyrogenitus (r. 913–959) mentions that the Serbian throne is inherited by the son, i.e., the first-born, though on one occasion there is a triumvirate in his enumeration of monarchs. The DAI's account about the Serbian ethnic settlement and establishment of several future principalities by the 10th century is considered as highly disputable: Serbia (roughly the later province of Rascia, including Bosnia; part of Zagorje - "hinterlands"); and Pagania, Zachlumia, Travunia (including Kanalitai) and Dioclea (part of Pomorje - "maritime").

Višeslav, the great-grandfather of Vlastimir and first Serbian monarch known by name, was a contemporary with Charlemagne (fl. 768–814). He directly held the hereditary lands of Tara, Piva and Lim. Constantine VI conquered the Sclaviniae (slavdom - "slav area") of Macedonia, situated to the south, in 785. Radoslav, then Prosigoj, succeeded Višeslav, and they ruled during the revolt of Ljudevit Posavski against the Franks (819–822). According to the Royal Frankish Annals, written in 822, Ljudevit went from his seat at Sisak to the Serbs, who controlled a great part of Dalmatia.

In the east, the Bulgarian Empire grew strong. In 805, khan Krum conquered the Braničevci, Timočani and Obotrites, to the east of Serbia, banished their tribal chiefs, and replaced them with administrators appointed by the central government. In 815, the Bulgarians and Byzantines signed a 30-year peace treaty. In 818 during the rule of Omurtag (814–831), the Braničevci and Timočani together with other tribes of the frontiers, revolted and seceded from Bulgaria because of an administrative reform that had deprived them much of their local authority. The Timočani left the societas (association, alliance) of the Bulgarian Empire, and sought, together with the Danubian Obotrites and Guduscani, protection from Holy Roman Emperor Louis the Pious (r. 813–840), and met him at his court at Herstal. The Timočani migrated into Frankish territory, somewhere in Lower Pannonia, and were last mentioned in 819, when they were persuaded by Ljudevit to join him in fighting the Franks. The Danubian Obotrites stayed in Banat, and resisted the Bulgarians until 824, when nothing more is heard of them. The khan sent envoys to the Franks and requested that the precise boundary be demarcated between them, and negotiations lasted until 826, when the Franks neglected him. The Bulgarians answered by attacking the Slavs that lived in Pannonia, and subjugated them, then sent ships up the Drava river, and, in 828, devastated Upper Pannonia, north of the Drava. There was more fighting in 829 as well, and by this time, the Bulgarians had conquered all of their former Slavic allies.

The Bulgarian state had a general policy of expansion in which they would first impose the payment of tribute on a neighboring people and the obligation of supplying military assistance in the form of an alliance (societas), leaving them internal self-government and local rulers, and when the need for this kind of relationship expired, they would terminate the self-government arrangement and impose direct and absolute power, integrating their neighbor fully into the Bulgarian political and cultural system.

Vlastimir succeeded his father, Prosigoj, as the archon of Serbia. According to Živković, the date of Vlastimir's accession was around 830. He united the Serbian tribes in the vicinity. The Serbs most likely consolidated due to alarm at the advance of Bulgaria towards their borders—a rapid conquest of neighbouring Slavs—in self-defence, and possibly sought to cut off the Bulgarian expansion to the south (Macedonia). Emperor Theophilos (r. 829–842) was recognized as the nominal suzerain (overlord) of the Serbs, and most likely encouraged them to thwart the Bulgarians. The thirty-year-peace treaty between the Byzantines and Bulgarians, signed in 815, was still in effect.

According to Constantine VII, the Serbs and Bulgaria had lived peacefully as neighbours until the invasion in 839 (in the last years of Theophilos). It is not known what exactly prompted the war, as Porphyrogenitus gives no clear answer; whether it was a result of Serbian-Bulgarian relations, i.e., the Bulgar conquest to the southeast, or a result of the Byzantine-Bulgarian rivalry, in which Serbia was an Imperial ally. It was not unlikely that the Emperor had a part in it; as he was at war with the Arabs, he may have pushed the Serbs to drive the Bulgaria from western Macedonia, which would benefit them both. According to J. Bury, this alliance would explain Malamir's action. Zlatarski supposes that the Emperor offered the Serbs complete independence in return.