Theoderic was the youngest son of Duke Theoderic II of Lorraine and Gertrude, daughter of Count Robert I of Flanders. After the murder of his cousin, Charles the Good, in 1127, Theoderic claimed the County of Flanders, but another cousin, William Clito, became count instead with the support of King Louis VI of France. William's politics and attitude towards the autonomy of Flanders made him unpopular, and by the end of the year Bruges, Ghent, Lille, and Saint-Omer recognized Theoderic as a rival count. Theoderic's supporters came from the Imperial faction of Flanders.