UDT may refer to:
- Underwater Demolition Team, American commando team and the precursor to the U.S. Navy SEALs
- Chadian Democratic Union (Union Démocratique Tchadienne), the second African political party ever created in Chad
- Timorese Democratic Union (União Democrática Timorense), a conservative political party in East Timor
- UDP-based Data Transfer Protocol, a high performance data transport protocol and its open source library
- Uitgeverij De Tijd, a former Belgian publisher
- Unarmed Defensive Tactics, as employed by various Police Forces as part of their training
- Undescended testes, a developmental abnormality in which the testes do not properly descend into the scrotum
- Union of Djibouti Workers, a trade union centre in Djibouti
- Universal Diver Training, a SCUBA diver training organization
- User-defined tag, a concept in Financial Information eXchange protocol to help customize messaging between various parties
- User-defined type, a concept in computer science that is generally synonymous with "record", "structure", "composite type" or "class"
- Yonkers UDT, a sport scuba diving club in Yonkers, New York, USA