The accession of George Terter I to the throne in 1280 did not alter matters, but, in 1281, Theodore Svetoslav was betrothed (in absentia) to a daughter of the sebastokratōr John I Doukas of Thessaly, as part of a diplomatic alliance.

It was only in 1285 that the Patriarch of Bulgaria Joakim III arrived in Constantinople and arranged for the release of Theodore Svetoslav, who was now supposed to marry a daughter of the high court official John Synadenos.

The reign of Theodore Svetoslav is connected with the internal stabilization and pacification of the country, the end of Mongol control of Tarnovo, and the recovery of portions of Thrace lost to the Byzantine Empire since the wars against Ivaylo of Bulgaria.

Theodore Svetoslav pursued a ruthless course of action, punishing all who stood in his way, including his former benefactor, Patriarch Joachim III, who was accused of treason and executed.

A new claimant appeared in the person of the sebastokratōr Radoslav Voïsil or Vojsil, from Sredna Gora, a brother of the former emperor Smilets, who was defeated, and captured by Theodore Svetoslav's uncle, the despotēs Aldimir (Eltimir), at Krăn in about 1301.

Theodore Svetoslav exchanged thirteen high-ranking Byzantine officers captured on Radoslav's defeat for his father George Terter I, whom he settled in a life of luxury in an unidentified city.

As a consequence of his victories, Theodore Svetoslav felt secure enough to move on to the offensive by 1303 and captured the fortresses of northeastern Thrace, including Mesembria (Nesebăr), Anchialos (Pomorie), Sozopolis (Sozopol), and Agathopolis (Ahtopol) in 1304.

The war ended with a peace treaty in 1307, cemented with a marriage between the widowed Theodore Svetoslav and Theodora, a daughter of the Co-emperor Michael IX Palaiologos.

His reassertion of central control over outlying provinces such as Vidin was probably peaceful, and the sparsely documented recovery of the banate of Severin from the Kingdom of Hungary must have been achieved during the dynastic struggles in that country.