Ger (Yiddish: גער, also Gur, adj. Gerrer) is a Polish Hasidic dynasty originating from the town of Góra Kalwaria, Poland, where it was founded by Yitzchak Meir Alter (1798–1866), known as the "Chiddushei HaRim". Ger is a branch of Peshischa Hasidism, as Yitzchak Meir Alter was a leading disciple of Simcha Bunim of Peshischa (1765–1827). Before the Holocaust, followers of Ger were estimated to number in excess of 100,000, making it the largest and most influential Hasidic group in Poland. As of 2024 the movement was based in Jerusalem and its membership is estimated at 15,000 families, most of whom live in Israel, making Ger the largest Hasidic dynasty in Israel. There are also well-established Ger communities in the United States and in Europe. In 2019, some 300 families of followers led by Shaul Alter, split off from the dynasty led by his cousin Yaakov Aryeh Alter.

In his early years, Yitzchak Meir Alter became a close disciple of Simcha Bunim of Peshischa, who preached the ideals of Talmudic rationalism and the pursuit of personal authenticity—later foundational tenets of Ger Hasidism. After Simcha Bunim died in 1827, Yitzchak Meir Alter was among his more radical supporters who followed Menachem Mendel of Kotzk (the Kotzker Rebbe) rather than Simcha Bunim's son, Avraham Moshe.

The Kotzker Rebbe continued in the ideological tradition of Peshischa, and after he died in 1859, most of his followers accepted Yitzchak Meir Alter as his successor rather than his son, Dovid. Yitzchak Meir Alter was living in Warsaw at the time and operated a Kotzker shtiebel (small congregation).

Shortly after accepting the role of Rebbe, Yitzchak Meir was appointed as the Av Beit Din of Góra Kalwaria (Yiddish: גער), where he established his own Hasidic court. After his death in 1866, his followers wanted his eighteen-year-old grandson, Yehudah Aryeh Leib Alter, to succeed him. When Yehuda Aryeh Leib refused to accept this position, most of the Hasidim became followers of the elderly Hasid Chanokh Heynekh HaKohen Levin, after whose death Yehudah Aryeh Leib acceded to the request of the Hasidim to become their next rebbe.

The Gerrer movement continued under the leadership of Yehudah Aryeh Leib and his eldest son and successor, Avraham Mordechai Alter, known as the "Imrei Emes".

In 1926, in an unusual move for Polish Hasidim, Avraham Mordechai established a yeshiva in Jerusalem, naming it for his father, the Sfas Emes. The first rosh yeshiva was Rabbi Nechemiah Alter, a brother of the Imrei Emes. The yeshiva is the flagship of the Ger yeshivas. Under the leadership of the fifth Gerrer Rebbe, Yisrael Alter, known as the "Beis Yisrael", the Ichud Mosdos Gur (or Union of Gerrer Institutions) was established as the responsible body for funding all the educational institutions affiliated with Ger in Israel. There are about 100 such institutions.^{[citation needed]} The Beis Yisrael helped rebuild the Ger movement after its virtual destruction in World War II.

Almost all Ger Hasidim living in pre-war Europe (approximately 100,000 Hasidim) were murdered by the Nazis in the Holocaust. Avraham Mordechai Alter, who managed to escape, set about the task of rebuilding the movement in the British Mandate of Palestine. Under its post-war leaders, the movement began to flourish again. With approximately 12,000 families, Ger is the third-largest Hasidic dynasty in the world today, comprising 9.2% of the world population of Hasidim. Large communities of Gerrer Hasidim exist in Israel, in Bnei Brak (2294 families / 19% of the Hasidic population), Ashdod (2218 families / 45%), and Jerusalem (1921 families / 12%), and a slightly smaller community of 1,027 families (6% of the Hasidic population) exists in the Borough Park neighborhood of Brooklyn, New York. Smaller communities with hundreds of families have also been established in Israel, such as Arad, Beit Shemesh, Kiryat Gat, Hatzor HaGlilit, Haifa, Dimona, Tel Aviv, and Petah Tikva. Internationally, hundreds of families reside in London, Antwerp, Zürich (where they are the largest Hasidic group), Manchester, Monsey, and Lakewood, with tens more living in Los Angeles, Queens, Montreal, Melbourne, and Chicago. Ger maintains a well-developed educational network of Talmud Torahs, yeshivas, and kollels, as well as Beis Yaakov schools for girls. The dynasty is the wealthiest in Israel, and its leaders dominate the Agudat Israel political party.

The group's headquarters is located in Jerusalem. During and after the British Mandate, the group's beth midrash was at the Sfas Emes Yeshiva, near Mahane Yehuda. Later on, the synagogue moved to Ralbach Street in the Geula neighborhood, and in the late 1990s, the Great Beth Midrash Gur was inaugurated on Yirmeyahu Street, near the Schneller Orphanage complex. In 2015, an extension to the building was begun, and on Rosh Hashanah 2018 (5779), the second wing of the Beth Medrash was inaugurated. In 2022, the building was finished when the two wings were joined, making it the largest synagogue in the world, with the main sanctuary seating up to 20,000, and having an area of approximately 35,000 m² (380,000 sq ft).