Who was the last Roman Emperor to rule a united Roman empire?

Answer 1

Theodosius I was the last emperor who ruled the entire empire. After his death (395), the there was a co-emperorship where is sons, Arcadius and Honorius, were in charge of the east and west respectively. This was a co-emperorship, not a division of the empire. There had been several co-emperorships prior to this. Historians say that the empire came to be divided after the death of Theodosius not because there was a decision to divided it, but because there was a loss of cohesion between the two parts. Soon after the accession of the two sons, the western part begun to crumble under the weight of the invasions by the Germanic peoples. It was unable to respond to the invasions because of infighting and intrigue for power struggles. East and west collaborated on fighting the invaders only once and this was a failure. In the end when there was a failure in governance in the west, the east interfered with the politics of the west. The last emperor in the west was deposed in 476, 81 years after the death of Theodosius.

Answer 2

Romulus Augustulus. A slight clarification is needed here. Romulus was the last emperor of the Western half of the empire. The last emperor to rule over the united Roman Empire was Theodosius I.