Sibir (sĭbērˈ) [key], former city, southeast of present-day Tobolsk, W Siberian Russia. Founded in the 11th or 12th cent., it became (early 16th cent.) the capital of the Tatar khanate of Sibir, which arose after the disintegration of the empire of the Golden Horde. The Cossack Yermak took the city of Sibir in 1581, thus marking the start of Moscow's conquest of what is now Siberia. The city was abandoned after the founding of Tobolsk in 1587.