Aliyah Bet (Hebrew: "Aliyah 'B'" – bet being the second letter of the Hebrew alphabet) is the code name given to illegal immigration by Jews, most of whom are refugees from Nazi Germany, and later Holocaust survivors, to Mandatory Palestine between 1934–48, in violation of the restrictions laid out in the British White Paper of 1939.

In modern-day Israel it has also been called by the Hebrew term Ha'apala (Hebrew: acension).

The Aliyah Bet is distinguished from the Aliyah Aleph ("Aliyah 'A'", Aleph being the first letter of the Hebrew alphabet) which refers to the limited Jewish immigration permitted by British authorities during the same period

The name Aliya B is also shortened name for Aliya Bilty Legalit (illegal immigration).

The British government limits Jewish immigration to Palestine with quotas, and following the rise of Nazism to power in Germany, illegal immigration to Palestine commences.

The illegal immigration is known as Aliyah Bet ("secondary immigration"), or Ha'apalah, and is organized by the Mossad Le'aliyah Bet, a branch of the paramilitary Jewish Defense Association (Haganah), as well as by the Irgun, established as a militant offshoot of the earlier and larger Haganah.

Immigration is done mainly by sea, and to a lesser extent overland through Iraq and Syria.

Beginning in 1939, Jewish immigration is further restricted, limiting it to 75,000 individuals for a period of five years after which immigration is to end completely.

During the Second World War and the years that follow until independence, Aliyah Bet becomes the main form of Jewish immigration to Palestine.Following the war, Berihah ("flight"), an organization of former partisans and ghetto fighters, is primarily responsible for smuggling Jews from Poland and Eastern Europe to the Italian ports from which they traveled to Palestine.Despite British efforts to curb the illegal immigration, during the 14 years of its operation, 110,000 Jews immigrate to Palestine.In 1945, reports of the Holocaust with its 6 million Jewish dead cause many Jews in Palestine to turn openly against the British Mandate, and illegal immigration escalates rapidly as many Holocaust survivors join the Aliyah.