Mardonius (märdō´nēəs), d. 479 BC, Persian general; son-in-law of Darius I. Darius sent him (492 BC) to retaliate against Eretria and Athens for aiding the Ionians in the Persian Wars, but his fleet was lost in a storm off Mt. Athos, and a Thracian tribe destroyed a large part of his army. He helped Xerxes I plan his invasion of Greece. Xerxes returned (480 BC) to Persia after his defeat at Salamis and left Mardonius in command in Greece. Mardonius was defeated and killed at Plataea.