In Greek mythology, Penthesilea (also spelled "Penthesilia") was an Amazonian queen, daughter of Ares and Otrera, sister of Hippolyte. Penthesilea killed Hippolyte with a spear when they were hunting deer. According to many accounts, this accident caused Penthesilea so much grief that she wished only to die, but, as a warrior and an Amazon, she must do so honorably and in battle. She therefore was easily convinced to join in the Trojan War, fighting on the side of the city's defenders.

She is said to have been killed by Achilles (or vice-versa, in rarer accounts) in battle. After her death, Achilles found himself awe-struck by her beauty, and when one of the Greek soldiers, Thersites, laughed at him for this, Achilles killed him. After that, more Greeks wanted Achilles to throw Penthesilea's remains into a river, and he eventually had to give way.

Alongside Penthesilea were twelve other Amazons, including Antibrote, Ainia, and Cleite, who was going to go to Troy with them but never made it.

She was succeeded as Queen of the Amazons by Antianara.