Aleph or alef (א) is the first letter of the Hebrew alphabet; is the equivalent letter of the Arabic alphabet. Aleph is comparable and related to the Greek alpha or letter A.

Aleph or alef may also refer to

• Alef, the concurrent programming language used in early editions of Plan 9;
• Alef, one of the main protagonists in SEGA's Shining Force;
• Aleph , a short story by author Georg Brandes;
• Aleph, a modern Buddhist religious group in Japan formerly known as Aum Shinrikyo;
• Aleph , an advanced system for inductive logic programming.
• ALEPH detector or ALEPH experiment, one of the four detectors of the Large Electron-Positron Collider (LEP) at CERN;
• aleph number or cardinality, the measurement of mathematical sets?
  • aleph null (), the cardinality of countable infinite sets;
  • aleph one (), the cardinality of certain uncountably infinite sets, possibly including the real numbers?
• Aleph One, an ongoing project related to the computer game Marathon;
• Aleph programming language;
• The Aleph, a short story by author Jorge Luis Borges.