Rabbi Yossi HaKohen, a Tanna of the second generation, lived at the end of the first century. He was one of the five leading disciples of Rabban Yochanan ben Zakkai, who called him "the chassid" (the pious one).

The Talmud relates that he studied Maaseh Merkavah, the secrets of Creation, with Rabbi Yehoshua, and that his teacher praised his mastery of hidden wisdom. Only one of his halachic opinions appears in the Mishnah, perhaps because of his piety, since the chassidim avoided drawing attention to themselves. He was so careful about the laws of Shabbat that "a document in his handwriting was never found in the hand of a gentile", lest it be carried on Shabbat.

Pirkei Avot quotes his teaching: "Let your fellow's money be as dear to you as your own; prepare yourself to study Torah, for it is not an inheritance; and let all your deeds be for the sake of Heaven." Asked by his teacher which good path a person should cleave to, he answered: "a good neighbour". He was also among those who came to comfort Rabban Yochanan ben Zakkai on the death of his son.

According to some sources he rests in Tiberias beside his teacher; others place his tomb in Alma, in Galilee.