The article examines the political relations between the Grand Duchy of Lithuania, especially Žemaitija as a constituent part, and Žemgala (Semigallia), from the beginning of the 1279 Žemgalian uprising against the Teutonic Order until the rule of Grand Duke Gediminas of Lithuania. The author tries to explain why Gediminas used the title of Duke of Žemgala in his letters of 1323, although in other cases, the title of the Lithuanian rulers does not include the name of Žemgala, and neither do other sources describing the territorial structure of the grand duchy mention Žemgala as part of it. Some historians have already argued that Žemgala was joined to the Grand Duchy of Lithuania in 1279. The article re-examines this argument, and tries to validate it. The cooperation of Lithuania (especially Žemaitija) with the Žemgalians during the war of 1279–1290 shows that the integration of Žemgala into the Lithuanian state was in fact its integration into Žemaitija during the war. The author concludes that this integration was not denied by the time Gediminas took power, despite the fact that the Teutonic Order had already initiated a new phase in the invasion of Žemgala. Gediminas used the title of Duke of Žemgala because he actually controlled most of Žemgala. A substantial part of it remained permanently within the Grand Duchy of Lithuania.