Aliyah Bet (Hebrew: עלייה ב’‎, “Aliyah ‘B'” – bet being the second letter of the Hebrew alphabet) was the code name given to illegal immigration by Jews, most of whom were refugees escaping from Nazi Germany,[1][2][3] and later Holocaust survivors, to Mandatory Palestine between 1920 and 1948,[2] in violation of the restrictions laid out in the British White Paper of 1939, which dramatically increased between 1939 and 1948. With the establishment of the State of Israel in May 1948, Jewish displaced persons and refugees from Europe began streaming into the new sovereign state.

In modern-day Israel it has also been called by the Hebrew term Ha’apala (Hebrew: הַעְפָּלָה‎, “Ascension”). 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 (Hebrew: עלייה בלתי-לגאלית‎, “illegal immigration”).