The Mossad LeAliyah Bet (Hebrew: המוסד לעלייה ב', lit. Institution for Immigration B) was a branch of the paramilitary organization Haganah in British Mandatory Palestine, and later the State of Israel, that operated to facilitate Jewish immigration to British Palestine. During the Mandate period, it was facilitating illegal immigration in violation of governmental British restrictions. It operated from 1938 until four years after the founding of the State of Israel in 1952. It was funded directly by the American Jewish Joint Distribution Committee (the JDC), and was not subject to the control of the Jewish Agency who operated their own Aliyah department headed by Yitzhak Rafael.

The Yishuv referred to legal immigration as "Aliyah Alef" (Alef is the first letter of the Hebrew alphabet, corresponding to the Latin A), whilst clandestine immigration was referred to as "Aliyah Bet" (Bet is the second letter, corresponding to the Latin B).