Inmarsat plc is a British satellite telecommunications company, offering global mobile services. It provides telephone and data services to users worldwide, via portable or mobile terminals which communicate with ground stations through twelve 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, is a constituent of the FTSE 100 Index and a financial (as well as technical) sponsor of Télécoms Sans Frontières.

The present company originates from the International Maritime Satellite Organization (INMARSAT), a non-profit intergovernmental organization established in 1979 at the behest of the International Maritime Organization (IMO)—the United Nations' maritime body—and pursuant to the Convention on the International Maritime Satellite Organization, signed by 28 countries in 1976. The organisation was created to establish and operate a satellite communications network for the maritime community. In coordination with the International Civil Aviation Organization in the 1980s, the convention governing INMARSAT was amended to include improvements to aeronautical communications, notably for public safety. The member states owned varying shares of the operational business.

In the mid-1990s, many member states were unwilling to invest in improvements to INMARSAT's network, especially owing to the competitive nature of the satellite communications industry, while many recognized the need to maintain the organization's older systems and the need for an intergovernmental organization to oversee public safety aspects of satellite communication networks. In 1998, an agreement was reached to modify INMARSAT's mission as an intergovernmental organization and separate and privatize the organization's operational business, with public safety obligations attached to the sale.

In April 1999, INMARSAT was succeeded by the International Mobile Satellite Organization (IMSO) as an intergovernmental regulatory body for satellite communications, while INMARSAT's operational unit was separated and became the UK-based company Inmarsat Ltd. The IMSO and Inmarsat Ltd. signed an agreement imposing public safety obligations on the new company. Inmarsat was the first international satellite organization that was privatized.

In 2005, Apax Partners and Permira bought shares in the company. The company was also first listed on the London Stock Exchange in that year. In March 2008, it was disclosed that U.S. hedge fund Harbinger Capital owned 28% of the company. In 2009, Inmarsat completed the acquisition of satellite communications provider Stratos Global Corporation (Stratos) and acquired a 19-percent stake in SkyWave Mobile Communications Inc., a provider of Inmarsat D+/IsatM2M network services which in turn purchased the GlobalWave business from TransCore. Inmarsat won the 2010 MacRobert Award for its Broadband Global Area Network (BGAN) service.