Hillel II, also known simply as Hillel was a Jewish communal and religious authority, circa 330 - 365 CE. He was the son and successor of Judah III. He is sometimes confused with Hillel the Elder, as the Talmud sometimes simply uses the name "Hillel". He is regarded as the creator of the Hebrew calendar.

In two instances is his name quoted in connection with important decisions in Jewish law: in one, Jose ben Abin expounds to him a law; in the other, Hillel cites a mishnah to establish a law (Yer. Ber. ii. 5a; Yer. Ter. i. 41a).

Rabbinic tradition ascribes to him an enactment which proved of incalculable benefit to his coreligionists of his own and of subsequent generations. To equalize the lunar with the solar year, and thereby render possible the universal celebration of the festivals on the days designated in the Bible, occasional intercalations of a day in a month and of a month in a year were required (see Calendar).

These intercalations were determined at meetings of a special commission of the Sanhedrin. But Constantius, following the tyrannous precedents of Hadrian, prohibited the holding of such meetings as well as the vending of articles for distinctively Jewish purposes.

The entire Jewish community outside the land of Israel depended on the calendar sanctioned by the Judean Sanhedrin; this was necessary for the proper observance of the Jewish holidays. However, danger threatened the participants in that sanction and the messengers who communicated their decisions to distant congregations. Temporarily to relieve the foreign congregations, Huna ben Abin once advised Rava not to wait for the official intercalation: "When you are convinced that the winter quarter will extend beyond the sixteenth day of Nisan declare the year a leap-year, and do not hesitate" (R. H. 21a). But as the religious persecutions continued, Hillel determined to provide an authorized calendar for all time to come, though by so doing he severed the ties which united the Jews of the Diaspora to their mother country and to the patriarchate.

The emperor Julian was gracious to Hillel, whom he honored on a number of occasions. In an autograph letter to him, Julian assured him of his friendship and promised to ameliorate further the condition of the Jews. Before setting out for the war with Persia, Julian addressed to the Jewish congregations a circular letter in which he informed them that he had "committed the Jewish tax-rolls to the flames," and that, "desiring to show them still greater favors, he has advised his brother, the venerable patriarch Julos, to abolish what was called the 'send-tax.'"

See also: Hillel