MCI may refer to the following:

• MCI WorldCom, Inc., a former American telecommunications company acquired by Verizon Communications
• Microwave Communications Inc., later MCI Communications, the corporation that operated as MCI from 1963–1998
• The airport code for the Kansas City International Airport in Kansas City, Missouri, USA
• Maine Central Institute, a boarding school in Maine, founded in 1866
• Martingrove Collegiate Institute, a high school located in Etobicoke, Toronto, Canada
• Media Control Interface/Multimedia Control Interface.
• Mild cognitive impairment, a neurological affliction often associated with Alzheimer's disease
• Motor Coach Industries, a company that builds buses
• MCI (audio), a company that manufactured multi-track audiotape recorders for professional studio recording
• Massachusetts Correctional Institution, a list of which can be found at List of Massachusetts state prisons
• Malicious Caller Identification, a type of enhanced telephone service
• Macroinvertebrate Community Index, a measure used in New Zealand for the environmental status of streams.