Dov Ber ben Avraham of Mezeritch (Yiddish: דֹב בּער פֿון מעזעריטש; died December 4, 1772 O.S.), also known as the Maggid of Mezeritch or Mezeritcher Maggid, was a disciple of Rabbi Israel ben Eliezer (the Baal Shem Tov), the founder of Hasidic Judaism, and was chosen as his successor to lead the early movement. Dov Ber is regarded as the first systematic exponent of the mystical philosophy underlying the teachings of the Baal Shem Tov, and through his teaching and leadership, the main architect of the movement.

He established his base in Mezhirichi (in Volhynia), which moved the centre of Hasidism from Medzhybizh (in Podolia), where he focused his attention on raising a close circle of disciples to spread the movement. After his death the third generation of leadership took their different interpretations and disseminated across appointed regions of Eastern Europe, rapidly spreading Hasidism beyond Ukraine, to Poland, Galicia and Russia.

His teachings appear in Magid Devarav L'Yaakov, Or Torah, Likutim Yekarim, Or Ha'emet, Kitvei Kodesh, Shemuah Tovah, and in the works authored by his disciples. His inner circle of disciples, known as the Chevraia Kadisha ("Holy Brotherhood"), included Rabbis Avraham HaMalach (his son), Nachum of Czernobyl, Elimelech of Lizhensk, Zusha of Hanipol, Levi Yitzchok of Berditchev, Boruch of Medzhybizh, Aharon (HaGadol) of Karlin, Chaim Chaykl of Amdur, Menachem Mendel of Vitebsk, Shmuel Shmelke of Nikolsburg, Shlomo Flam (the Lutzker Maggid), Asher Zebi of Ostrowo, Zev Wolf of Zhitomyr, and Shneur Zalman of Liadi.

The most common transliterations are Dov-Ber, Dov Baer and DovBer; rarely used forms are Dob Ber or Dobh Ber, which often depend on the region in Eastern Europe where Jews resided and hence the influence of the local Yiddish dialects. The name דוב-בער Dov-Ber is traceable back to the Hebrew: דב, romanized: dov, lit. 'bear', and Yiddish: בער, romanized: ber, lit. 'bear'. It is thus an example of a bilingual tautological name.

Dov Ber of Mezeritch was known as the Maggid—"Preacher" or literally "Sayer," one who preaches and admonishes to go in God's ways—of Mezritsh (the German form Meseritz is sometimes used instead of Mezeritch). Towards the end of his life he was also known as the Maggid of Rivne, the town where he was buried.

Dov Ber was born in Lokachi, Volhynia in 1710, according to the Jewish Encyclopedia, though his year of birth is unknown and some sources place it around 1700. Little is known about him before he became a disciple of the Baal Shem Tov. A Hasidic legend states that, when he was five years old, his family home burst into flames. On hearing his mother weeping, he asked: "Mother, do we have to be so unhappy because we have lost a house?" She replied that she was mourning the family tree, which was destroyed, and is traced to King David by way of Rabbi Yohanan, the sandal-maker and master in the Talmud. The boy replied: "And what does that matter! I shall get you a new family tree which begins with me!"

When he was young, he reportedly lived in great poverty with his wife. One legend relates that when a child was born, they had no money to pay the midwife. His wife complained and the Maggid went outside to "curse" Israel. He went outside and said: "O children of Israel, may abundant blessings come upon you!" When his wife complained a second time, he went outside again and cried: "Let all happiness come to the children of Israel—but they shall give their money to thorn bushes and stones!" The baby was too weak to cry, and the Maggid sighed rather than "cursing". Immediately the answer came, and a voice said: "You have lost your share in the coming world." The Maggid replied: "Well, then, the reward has been done away with. Now I can begin to serve in good earnest."

He is buried in Hannopil, beside Zusha of Hanipol.