The Boeing Starliner (or CST-100) is a spacecraft designed to transport crew to and from the International Space Station (ISS) and other low-Earth-orbit destinations. Developed by Boeing under NASA's Commercial Crew Program (CCP), it consists of a reusable crew capsule and an expendable service module.

Slightly larger than the Apollo command module or SpaceX Crew Dragon, but smaller than the Orion capsule, the Starliner can accommodate a crew of up to seven, though NASA plans to fly no more than four. It can remain docked to the ISS for up to seven months and is launched on an Atlas V N22 rocket from Cape Canaveral Space Launch Complex 41 in Florida.

In 2014, NASA awarded Boeing a US$4.2 billion fixed-price contract to develop and operate Starliner, while SpaceX received $2.6 billion to develop and operate Crew Dragon. By February 2025, Boeing's effort had exceeded its budget by at least $2 billion.

Originally planned to be operational in 2017, Starliner has been repeatedly delayed by problems in management and engineering. The first uncrewed Orbital Flight Test in December 2019 was deemed a partial failure, leading to a second Orbital Flight Test in May 2022. During the Crew Flight Test, launched in June 2024, the Starliner's thrusters malfunctioned on approach to the ISS, and NASA concluded that it was too risky to return its astronauts to Earth aboard the spacecraft, so it returned uncrewed in September 2024.

As the Space Shuttle program drew to a close, NASA sought to foster the development of new spaceflight capabilities. Departing from the traditional model of government-owned and operated spacecraft, NASA proposed a different approach: companies would own and operate spacecraft while NASA would act as a customer, purchasing flights as needed. NASA offered funding to support the development of these new vehicles, but unlike previous cost-plus contracts these new contracts would be fixed-price, placing the financial risk of cost overruns on the companies themselves.

Boeing had an extensive history of developing vehicles for space exploration, having built the first stage (S-IC) of the Saturn V rockets, assembling the Lunar Roving Vehicles, and serving as the prime contractor for the US Orbital Segment of the ISS since 1993. With this record and deep expertise, Boeing was seen as well-positioned to compete for the commercial spaceflight contracts.

In 2010, Boeing unveiled its entry into NASA's Commercial Crew Program competition: the CST-100. The company received initial funding of $18 million in the first round of the Commercial Crew Development (CCDev 1) program to support Starliner's development. Additionally, United Launch Alliance, a joint venture between Boeing and Lockheed Martin, secured $6.7 million to develop the Emergency Detection System to allow its Atlas V rocket to be human-rated to launch Starliner. At the time, Boeing expressed optimism that the Starliner could be operational as early as 2015, contingent upon timely approvals and funding.

In October 2011, NASA announced that the Orbiter Processing Facility-3 at Kennedy Space Center would be leased to Boeing for manufacture and test of Starliner, through a partnership with Space Florida.