Masistes (Old Persian 𐎶𐎰𐎡𐏁𐎫, Maθišta; Greek Μασίστης, Masístēs; Old Iranian *Masišta; died c. 478 BC) was a Persian prince of the Achaemenid Dynasty, son of king Darius I (reign: 520-486 BC) and of his wife Atossa, and full brother of king Xerxes I (reign: 486-465 BC). He was satrap (governor) of Bactria during his brother's reign, where he attempted to start a revolt in 478 BC.

Masistes was one of the six chief marshals of the Greek campaigns of Xerxes (480-479 BC). Along with general Mardonius, he commanded the army column that crossed Thrace along the coast; however, he was almost completely absent during the course of the war, including the battles of Salamis and Plataea.

He reappeared close to the end of the war, when he fought in the Battle of Mycale (479 BC). According to Herodotus, the battle was just about to take place at sea, but the Persians decided to come ashore in Ionia, Asia Minor, and fight on land. The Greek and the Persian armies finally fought in Mycale, and the Persians were completely defeated. Commanders Tigranes and Mardontes died during the battle, but Artayntes and Itamithres, as well as Masistes, managed to escape. According to Herodotus, on their way to Sardis, where the king stayed, Masistes accused Artayntes of cowardice and blamed him of being "worse than a woman". In response, Artayntes unsheathed his sword and tried to kill Masistes, but was stopped by a Carian named Xenagoras of Halicarnassus.

Masistes' family stayed near the battle, in Sardis, Lydia, which was a common practice of the nobility of the Persian Empire. In Herodotus' Histories, he comments that while Masistes was fighting, his brother, king Xerxes, seduced Masistes' wife (see below).

Masistes is one of the main characters in a bloody episode in Herodotus' Histories (IX 108–110). Herodotus relates that, after his defeat in Greece, Xerxes fell in love with the wife of Masistes, but failed in all attempts to win her favour. In order to be closer to his sister-in-law, Xerxes married his son Darius to his niece Artaynte, a daughter of Masistes. The wedding was celebrated in Sardis, but when the court returned to Susa, Xerxes decided that he desired the daughter rather than the mother, and Artaynte became his lover. One day, Xerxes visited Artaynte wearing a robe that had been woven by queen Amestris. He offered Artaynte to grant her whatever wish she desired. She chose the robe, and when she wore it in public, the affair was discovered.

When Amestris heard the news she sought revenge, not from Artaynte, strangely enough, but from her mother, the wife of Masistes. It was customary in Persia that on the king's birthday he had to grant all the wishes he was asked, so Amestris waited until that date to ask Xerxes to have Masistes' wife killed. Under Amestris' orders, Masistes' wife was tortured and mutilated: her breasts, nose, ears, lips and tongue were cut out. In the meantime, Xerxes offered Masistes the hand of one of his daughters. Masistes refused, and when he saw his mutilated wife he fled to Bactria to start a revolt; however, he was intercepted on his way by troops sent by the king, and put to death along with his followers and his three sons.

The tale refers to an ill-fated attempt of Masistes to lead a revolt against his brother Xerxes. Although the exact date is unknown, it could not have happened too long after the battle of Mycale (479 BC), perhaps in 478 BC.

It was originally considered that Xerxes, affected by his defeats in Greece, became involved in harem intrigues that would cause turmoil in the court and eventually result in the decadence and ruin of the Empire. This view is widely criticised by more modern authors. Many of the episode's themes appear in many legendary tales of Persian origin - the plot is the same, only the names of the characters are changed. Thus, although the tale is based on Persian oral accounts of Masistes' revolt, it is impossible to consider it as a historical fact and to derive literal conclusions from it.