Rabbi Yaakov Yisrael ben Tzvi, the Yaavetz, was born in Altona on 15 Sivan 5458 (1698), the son of the Chacham Tzvi, and named after his grandfather, Rabbi Yaakov Zak; he also descended from Rabbi Eliyahu Baal Shem of Chelm. In 1716 he married Rachel, daughter of his teacher Rabbi Mordechai HaKohen Katz, son of Rabbi Naftali Katz, and lived in Breslau.

In 1728 he was invited to the rabbinate of Emden, whence the name by which he is known; out of firmness of character and a wish for independence, he resigned four years later and settled in Altona near Hamburg, where he refused any official post and set up his own printing press.

He is best known for his siddur, Beit Yaakov (Siddur Yaavetz), rich in laws and commentary and still widely used. He wrote thirty-one works in all, seven of them published posthumously: responsa (She'elat Yaavetz), Lechem Shamayim on the Mishnah, Mor uKetziah on the Shulchan Aruch, and an autobiography, Megillat Sefer. A fierce opponent of the hidden followers of Shabbetai Tzvi, he waged a long controversy against Rabbi Yonatan Eybeschütz over his amulets. His son, Rabbi Meshullam Shlomo, was rabbi of the Hambro Synagogue in London.

He passed away in Altona on 30 Nisan 5536 (1776) and rests in the city's Jewish cemetery.