ACI may refer to:
- Accusative and infinitive aka Accusativus cum infinitivo, a construction in Latin grammar
- Access Control Item, used for Access Management. See also Access Control List.
- Aci Castello (Sicilian: Jaci Casteddu), a city and comune in the Province of Catania in Sicily, Italy
- Adjacent-channel interference, power from a signal in an adjacent channel
- Adriatic Croatia International Club, a chain of yachting marinas (referred to as ACI Marinas) operating in the Croatian part of the Adriatic sea
- Agencia Central de Intelligencia, Colombia's newly proposed national intelligent agency.
- Agincourt Collegiate Institute a secondary school located in Toronto, Canada
- Air Cargo Inc, an organization that provides information to the freight forwarding industry
- Air Crash Investigation, an alternate name for Mayday (TV series)
- Airports Council International, international trade group of the world's commercial aviation industry, based in Montreal, Canada
- Albertson College of Idaho, now known as the College of Idaho, a small liberal-arts college in Idaho
- Alderney Airport, from its IATA airport code
- American Collegiate Institute - Izmir Amerikan Lisesi, located at İzmir, Turkey
- American Competitiveness Initiative, a federal program intended to help America maintain competitiveness
- American Concrete Institute
- American Constitutional Initiative, a non-profit organization that promotes progressive change
- Arch Coal inc., A major U.S. coal mining company
- Aurora Cable Internet, a digital cable television, cable internet and VOIP provider in Aurora, Ontario
- Autologous chondrocyte implantation, a medical treatment for articular cargilage damage.
- Automatic Car Identification, a railroad barcode system that preceded Automatic Equipment Identification
- Automobile Club d'Italia, a member of the Fédération Internationale de l'Automobile
- ACI, the ICAO code for Aircalin
- ACI (Agence Congolaise d'Info) Congo news agency
Aci is also the shortened form of the local name of the Sicilian city Acireale