Attalus III ( Άτταλος Γ' ) was the last Attalid king of Pergamon, ruling from 138 BC to 133 BC.

He succeeded Attalus II, although their relationship, if any, is unknown. Attalus III had little interest in ruling Pergamon, devoting his time to studying medicine, botany, gardening, and other pursuits. He had no children or heirs of his own, and in his will he left the kingdom to the Roman Republic. Tiberius Gracchus requested that the treasury of Pergamon be opened up to the Roman public, but the Senate refused this.

Not everyone in Pergamon accepted Rome's rule. Aristonicus, who claimed to be Attalus' brother as well as the son of Eumenes II, an earlier king, led a revolt among the lower classes. The revolt was put down in 129 BC, and Pergamon was divided among Rome, Pontus, and Cappadocia.

Preceded by: Attalus II
Attalid Ruler
Succeeded by:Eumenes III