Ziaelas (Greek: Ζιαήλας; lived c. 265 BC – 228 BC, reigned c. 254 BC – 228 BC), third king of Bithynia, was a son of Nicomedes I and Ditizele.
After Nicomedes I died, his second wife Etazeta of Bithynia became ruler on behalf of her infant sons. The grown-up Ziaelas, excluded from the throne, had previously fled to Armenia and taken refuge at the court of King Arsames I in Sophene. On his father's death he immediately endeavoured to regain his rights by force, and returned, aided by some Galatians. Although Etazeta was supported by neighbouring cities and Antigonus II Gonatas, Ziaelas rapidly conquered first part, then all, of Bithynia, forcing Etazeta and her sons to escape to the court of Antigonus III Doson in about 254 BC.
He was succeeded by his son Prusias I about 228 BC after being killed by the Galatian Gauls. Like his father and his grandfather he also founded a new city named after him, Ziela, but the location of the city is unknown. His daughter, born c. 245 BC, married Antiochus Hierax.