Inmarsat plc (LSE: ISAT) is a British satellite telecommunications company, offering global, mobile services. It provides telephony and data services to users worldwide, via portable or mobile terminals which communicate to ground stations through eleven geostationary telecommunications satellites. Inmarsat's network provides communications services to a range of governments, aid agencies, media outlets and businesses with a need to communicate in remote regions or where there is no reliable terrestrial network. The company is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index as of December 2011.

Aside from its commercial services, Inmarsat provides global maritime distress and safety services (GMDSS) to ships and aircraft at no charge, as a public service.

Services include traditional voice calls, low-level data tracking systems, and high-speed Internet and other data services as well as distress and safety services. Other services provide mobile Integrated Services Digital Network (ISDN) services used by the media for live reporting on world events via videophone.

Newer Inmarsat services use an IP technology that features an always-on capability where the users are only charged for the amount of data they send and receive, rather than the length of time they are connected. This applies specifically to BGAN and MPDS.

The first (F1) and second (F2) of Inmarsat's most recent series of satellites, known as the "I4" satellites, were launched in June and November 2005. The third and final satellite (F3) was launched from the Cosmodrome in Kazakhstan on the 18 August 2008. These were the largest commercial telecommunications satellites ever launched. Each satellite is equipped with a global beam, 19 regional spot beams, and over 200 narrow spot beams.

In addition to its own satellites, Inmarsat has a collaboration agreement with ACeS regarding handheld voice services.