Simon Thassi was a Jewish leader of the Hasmonean dynasty, serving as high priest, military commander, and ruler of Judea. The second son of the Hasmonean patriarch Mattathias and one of the Maccabean brothers, he assumed leadership after his brother Jonathan Apphus was captured by the Seleucid general Diodotus Tryphon. Simon played a central role in consolidating Hasmonean rule: he strengthened Judea's fortifications, expelled the Seleucid garrison from Jerusalem, and expanded Jewish settlement, laying the foundation for the Hasmonean state. His rule marked the beginning of effective Jewish independence.