Hastings' Dictionary of the Bible 
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
( ῎Ατταλος , a Macedonian name of uncertain signification), a king of Pergamus in the time of the Jewish prince Simon ( Maccabees 11:22), and, as would appear from the connected circumstances, about B.C. 139; a closer determination of the date depends upon the year of the consul Lucius (q.v.), named in the same connection ( Acts 14:16), which is itself doubtful. As Attalus was the name of three kings of Pergamus, who reigned respectively B.C. 241-197, 159-198 (Philadelphus), 138-133 (Philometor), and were all faithful allies of the Romans (Liv. 45:13), it is uncertain whether the letters sent from Rome in favor of the Jews ( 1 Maccabees 15:22) were addressed to Attalus II (Polyb. 25:6; 31:9; 32:3, 5, 8, etc., 25 sq.; Just. 35:1; 36:4, 5; App. Milh. 62), known as the "friend of the Roman people" (Strabo, 13, p. 624), or Attalus III (Philometor), the nephew and successor of Attalus II, and son of Eumenes II, who ascended the throne B.C. 138, and by whose testament the kingdom of Pergamus passed over (B.C. 133) into the hands of the Romans (Justin, 36:4; Flor. 2:20; Strabo, 13:624). Josephus quotes a decree of the Pergamenes in favor of the Jews (Ant. 14, 10. 22) in the time of Hyrcanus, about B.C. 112 (comp. Revelation 2:12-17). — Smith, s.v.
International Standard Bible Encyclopedia 
at´a - lus : King of Pergamum, mentioned in 1 Macc 15:22 among the kings to whom was sent an edict ( Ant. , Xiv , viii, 5) from Rome forbidding the persecution of the Jews. See Attalia .