king of Pergamum, in northwest Anatolia, from 159 BC until his death.

He was the second son of King Attalus I Soter (reigned 241–197) and brother of Eumenes II (reigned 197–159), whom he succeeded. Before his accession he had been a loyal assistant to his brother, commanding the Pergamene forces that were fighting beside the Romans in campaigns in Galatia (189) and Greece (171). Attalus' frequent ambassadorial missions to Rome earned him a favourable reputation there, and he maintained close ties with the Romans after becoming king. They helped him hold his own in his struggle (156–154) against Prusias II, the aggressive king of Bithynia in northern Anatolia, and they conspired with him to aid the pretender Alexander Balas in overthrowing the Seleucid king Demetrius I in 150.