Inmarsat 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 fifteen geostationary telecommunications satellites.

Inmarsat's network provides communications services to a range of governments, aid agencies, media outlets and businesses (especially in the shipping, airline and mining industries) with a need to communicate in remote regions or where there is no reliable terrestrial network. The company was listed on the London Stock Exchange until it was acquired by Connect Bidco, a consortium consisting of Apax Partners, Warburg Pincus, the CPP Investment Board and the Ontario Teachers' Pension Plan, in December 2019.

On 8 November 2021, Inmarsat's owners and Viasat announced the purchase of Inmarsat by Viasat. The acquisition was completed in May 2023.

The present company originates from the International Maritime Satellite Organization (INMARSAT), a non-profit intergovernmental organisation 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. The main offices were originally located in the Euston Tower, Euston Road, London.

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 recognised the need to maintain the organisation's older systems and the need for an intergovernmental organisation to oversee public safety aspects of satellite communication networks. In 1998, an agreement was reached to modify INMARSAT's mission as an intergovernmental organisation and separate and privatise the organisation'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 organisation that was privatised.

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-per cent 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.

Inmarsat at first provided services using Marisat and MARECS, which were launched by the US Navy and ESA respectively. In the early 1990s Inmarsat launched its first dedicated satellite constellation, Inmarsat-2. These satellites provided the Inmarsat-A service for maritime uses. Between 1996 and 1998 Inmarsat's second constellation, Inmarsat-3, was launched. Consisting of five geostationary L-band satellites the constellation provides the Inmarsat-B and Inmarsat-C services, primarily providing low bandwidth communications and safety services for global shipping. Following privatisation in 1999 Inmarsat developed and launched the first satellite communications system offering global coverage, BGAN. This service was provided initially through the three Inmarsat-4 satellite launched between 2005 and 2008, and was then extended with the addition of Alphasat in 2013. In the 2010s, Inmarsat began development of the High Throughput Satellite (HTS) constellation Global Xpress, operating in the Ka-band portion of the spectrum. Global Xpress, launched in 2015, offers global satellite capacity to various markets including shipping and aviation. Global Xpress also marks a significant expansion of Inmarsat's commercial operations in the aviation markets. In 2017, Inmarsat launched its first S-band satellite, intended to provide (in association with an LTE ground network) inflight internet access across Europe. In March 2018, Inmarsat partnered with Isotropic Systems to develop a state-of-the-art, all electronic scanning antenna intended to be used with the Global Xpress network.