Yehudah Aryeh Leib Alter (Hebrew: יהודה אריה ליב אלתר, 15 April 1847 – 11 January 1905), also known by the title of his main work, the Sfas Emes (Ashkenazic Pronunciation) or Sefat Emet (שפת אמת‎) (Modern Hebrew), was a Hasidic rabbi who succeeded his grandfather, Rabbi Yitzchak Meir Alter, as the Av beis din (head of the rabbinical court) and Rav of Góra Kalwaria, Poland (known in Yiddish as the town of Ger), and succeeded Rabbi Chanokh Heynekh HaKohen Levin of Aleksander as Rebbe of the Gerrer Hasidim.

[1] Orphaned of both parents, he was brought up by his grandparents, Rabbi Yitzchak Meir Alter (known as the Chiddushei Harim) and his wife.

When his grandfather, Yitzchak Meir, died in 1866, many of the Gerrer Hasidim sought to bestow the mantle of leadership upon eighteen-year-old Yehudah Aryeh Leib.

A year later, a fire consumed the Chassidic buildings in Góra Kalwaria, including Alter's home and hisBeth midrash.

His output was prodigious, and his works (all entitled Sfas Emes) deal with the Talmud, the ethics of the Midrash, and mysticism of the Zohar.

His Torah homilies as delivered to his hasidim, and arranged according to the weekly parashah and the festivals, were the first to be published posthumously under the name Sfas Emes (שפת אמת).