Shimon HaTzadik (Simeon the Just) was High Priest in Jerusalem in the Second Temple period. Rabbinic sources and Josephus identify him as Shimon son of Choniyo son of Yaddua, a descendant of Yehoshua ben Yehotzadak, the High Priest of the return from Babylon. Historians debate which of two High Priests of that name, grandfather and grandson in the third century BCE, he was.

According to the Mishnah he was one of the last members of the Great Assembly (Anshei Knesset HaGedolah). He is the first sage named in the Mishnah and the teacher of Antigonus of Sokho; according to the Rambam he was a disciple of Ezra the Scribe. Pirkei Avot records his teaching: "The world stands on three things: on the Torah, on the Temple service, and on acts of loving-kindness."

The Talmud relates that he served as High Priest for forty years and prepared two red heifers. During his tenure the lot "for God" always came up in the right hand on Yom Kippur, the crimson thread turned white and the western lamp of the Menorah stayed lit; after his death these signs were no longer constant. The Talmud also tells that, wearing his priestly garments, he went out to meet Alexander the Great, who descended from his chariot and bowed before him; Josephus places this meeting in the days of his grandfather Yaddua.

In the year of his death he foretold it: every Yom Kippur an old man dressed in white had entered and left the Holy of Holies with him, but that year he was dressed in black and did not come out. He fell ill after the festival and died seven days later, which is the source of the tradition placing his passing on 29 Tishrei.

According to a tradition attested since the Middle Ages, he is buried in the cave that bears his name, in the Shimon HaTzadik neighborhood north of Jerusalem's Old City. The site was purchased in 1876 by the Rishon LeZion Rabbi Avraham Ashkenazi. Hilloulot are held there on 29 Tishrei and on Lag BaOmer, when Jerusalemites customarily bring young boys for their first haircut (chalakah).