Seleucus III Soter, called Seleucus Ceraunus (Greek: Σέλευκος Γ΄ ὁ Σωτήρ, ὁ Κεραυνός; c. 243 BC – April/June 223 BC, ruled December 225 – April/June 223 BC),^[1] was a ruler of the Hellenistic Seleucid Kingdom, the eldest son of Seleucus II Callinicus and Laodice II.

His birth name was Alexander and he was named after his great uncle the Seleucid official Alexander. Alexander changed his name to Seleucus after he succeeded his father as King. Seleucus's rule was troubled & unstable, during his tenure he unsuccessfully continued his father's war in Asia Minor against Attalus I of Pergamon of Pergamum. Seleucus was seen as being energetic & full of ambition like his father was. However he lacked character & was usually very sickly, making him mostly unable to fulfill his ambitious ideas. He however, in the eyes of many did not suite his position. He was not very well liked by many as he did not fit their definition of a proper King, as much as he did try to prove the opposite. After a brief reign of less than two years (225–223 BC), he was assassinated in Anatolia by members of his army, possibly at the instigation of his powerful minister Hermeias.^[1] His official byname Soter means "Saviour", while his nickname Ceraunus means "Thunderbolt".