Arsinoe II ( 316-270 BC), queen of Thrace and later co-ruler of Egypt with her brother and husband Ptolemy II of Egypt.

Arsinoe II was first married to King Lysimachus of Thrace, to whom she bore three sons. After his death in battle in 281 BC, she fled to Cassandrea and married her half-brother Ptolemy Keraunos. This proved to be a serious misjudgement, as Ptolemy Ceraunus promptly killed two of her sons; the third was able to escape. Arsinoe fled again, this time to Alexandria, Egypt.

In Egypt, she probably instigated the accusation and exile of her brother Ptolemy II's first wife, Arsinoe I of Egypt . Arsinoe II then married her brother; as a result, both were given the epithet "Philadelphoi" ("Brother-Loving") by the scandalized Greeks. Arsinoe II shared all of her brother's titles and apparently was quite influential, having towns dedicated to her, her own cult (as was Egyptian custom), and appearing on coinage. Apparently, she contributed greatly to foreign policy, including Ptolemy's victory in the First Syrian War (274-271 BC) between Egypt and the Seleucid Empire in the Middle East. After her death Ptolemy II continued to refer to her on official documents, as well as supporting her coinage and cult.