The Imperium is a series of consecutive fictional galactic empires in the Traveller universe.

• The First Imperium (or Ziru Sirka) was an empire of the Vilani.
• The Second Imperium (or Rule of Man) was formed by the Solomani as after they conquered the First. The Rule of Man collapsed due to the weight of stagnation that it had absorbed from the Ziru Sirka.
• The Third Imperium arose after a period known as The Long Night, when a small federation of planets known as the Sylean Federation re-absorbed the worlds of the previous empires. The early years of the third Imperium is the setting for T4 Marc Miller's Traveller . A well established Third Imperium is the setting for Classic Traveller and Traveller20. MegaTraveller and are set after the collapse of the Third Imperium. GURPS Traveller is set in alternate timeline where the Third Imperium did not collapse.