Aliyah Bet (Hebrew: 'עלייה ב), meaning "Aliyah 'B'" (bet being the second letter of the Hebrew alphabet) was the code name given to illegal immigration by Jews to the British Mandate of Palestine in violation of British restrictions against such immigration.

It was distinguished from Aliyah Aleph ("Aliyah 'A'") (Aleph is the first letter of the Hebrew alphabet): the limited Jewish immigration British authorities did allow during the mandatory period.

In modern day Israel it has also been called the Ha'apala (Hebrew: העפלה), which is also a Hebrew term used for the immigration by Jews to Palestine in violation of British restrictions against such immigration. During Ha'apala, several Jewish organizations worked together to facilitate immigration beyond the established quotas. As persecution of Jews intensified in Europe during the Nazi era, the urgency driving the immigration also became more acute. Those who participated in the immigration efforts consistently refused to term it "illegal", instead calling it "clandestine."