ACH or Ach may refer to:

• IATA airport code for St. Gallen–Altenrhein Airport in Switzerland

• ACH Food Companies, Inc., American subsidiary of Associated British Foods
• ACH Network, American electronic fund clearing house
• Automotive Components Holdings, LLC, a Ford-managed temporary company

• Adelaide Children's Hospital, former component of the Women's and Children's Hospital, Adelaide, South Australia
• Akron Children's Hospital, Akron, Ohio, U.S.
• Alberta Children's Hospital, Calgary, Alberta, Canada
• Arkansas Children's Hospital, Little Rock, Arkansas, U.S.

• Accion contra el hambre, part of the Action Against Hunger network
• Association for Computers and the Humanities

• Ach (surname)
• Ach., taxonomic author abbreviation for Erik Acharius (1757–1819), Swedish botanist
• ACH (wrestler), American professional wrestler Albert C. Hardie, Jr (born 1987)

• Aach (toponymy), a term for "river" in German hydronymy
  • Ach (Ammer), a tributary of the Ammer, Bavaria, Germany
  • Ach (Blau), a tributary of the Blau, Baden-Württemberg, Germany
  • Bregenzer Ach, a tributary of Lake Constance, Austria
  • Dornbirner Ach, a tributary of Lake Constance, Austria
  • Friedberger Ach, a tributary of the Lech, Bavaria, Germany
• Ach, Iran, a village in Qazvin Province

• Acetone cyanohydrin, an organic compound
• Acetylcholine (ACh), a neurotransmitter
• Aluminium chlorohydrate, a group of water-soluble, specific aluminium salts

• AC Horsens, a Danish football club
• Allan Cup Hockey, a Canadian senior ice hockey league

• Achnashellach railway station, Scotland, National Railway code ACH

• Acholi dialect, ISO 639 code ach
• Advanced Combat Helmet, in the United States Army
• Air changes per hour, a measure of ventilation
• Analysis of competing hypotheses, a methodology
• Automated clearing house, a network for processing transactions

• Aboriginal Cultural Heritage Act 2021, or ACH Act, in Western Australia