Shimon bar Yochai (Zoharic Aramaic: שמעון בר יוחאי, Šimʿon bar Yoḥay) or Shimon ben Yochai (Mishnaic Hebrew: שמעון בן יוחאי), also known by the acronym Rashbi, was a 2nd-century tanna or sage of the period of Roman Judaea and early Syria Palaestina. He was one of the most eminent disciples of Rabbi Akiva. The Zohar, a 13th-century foundational work of Kabbalah, is ascribed to him by Kabbalistic tradition, but this claim is universally rejected by modern scholars.

In addition, the essential legal works called the Sifre and Mekhilta of Rabbi Shimon ben Yochai are attributed to him (not to be confused with the Mekhilta of Rabbi Ishmael, of which much of the text is the same). In the Mishnah, where he is the fourth-most mentioned sage, he is referred to as simply "Rabbi Shimon" except in Hagigah 1:7. In baraitas, midrash, and gemara, his name occurs either as Shimon or as Shimon ben Yochai. An 8th-century pseudonymous attribution of divine revelations to Shimon by the angel Metatron is also known, called The Secrets of Rabbi Simon ben Yohai.

According to modern legend, he and his son, Eleazar ben Simeon, were noted Kabbalists. Baruch Kranz denies this claim. Both figures are held in unique reverence by Kabbalistic tradition. By tradition, they were buried in the same tomb in Meron, Israel, which is visited by thousands year-round.

Shimon was born in the Galilee. He was one of the principal pupils of Rabbi Akiva, under whom he studied 13 years at Bnei Brak.

Berakhot 28a relates that Shimon had previously studied at Yavne, under Gamaliel II and Joshua ben Hananiah, and that he was the cause of the infamous quarrel that broke out between these two leaders. However, this statement is somewhat chronologically difficult, considering that about forty-five years later, when Akiva was thrown into prison, Shimon's father was still alive. Zecharias Frankel thus concluded the account in Berakhot 28a is spurious.

In the Jerusalem Talmud, in Terumot 46b and Sanhedrin 1:19a, Shimon's acuteness was tested and recognized by Aqiva when he first came to him; of all his pupils Aqiva ordained only Rabbi Meir and Shimon. Conscious of his merit, Shimon felt hurt at being ranked after Meir, and Aqiva was compelled to soothe him with soft words.

During Akiva's lifetime, Shimon was occasionally found at Sidon, where he seems to have shown great independence in his halakhic decisions.

The following incident is recorded, illustrating his wit and piety: A man and his wife, childless despite ten years of marriage, appeared before Shimon at Sidon to secure a divorce. Observing that they loved each other and not being able to refuse a request that agreed with rabbinical law, Shimon told them that as a feast marked their wedding, they should mark their separation in the same way. The result was that both changed their minds and, owing to Shimon's prayer, God granted them a child.