Lothair II or Lothair III of Supplinburg (before June 9, 1075 – December 4, 1137) is Duke of Saxony (1106), King of Germany (1125), and Holy Roman Emperor from 1133 to 1137.

The son of Count Gebhard of Supplinburg, his reign is troubled by the constant intriguing of Frederick II, Duke of Swabia and Duke Conrad of Franconia.

He dies while returning from a successful campaign against the Normans in southern Italy.