The Commercial Crew Program (CCP) provides commercially operated crew transportation service to and from the International Space Station (ISS) under contract to NASA, conducting crew rotations between the expeditions of the International Space Station program. The American space manufacturer SpaceX began providing service in 2020, using Crew Dragon, and NASA plans to add Boeing when Starliner becomes operational no earlier than 2026. NASA has contracted for six operational missions from Boeing and fourteen from SpaceX, ensuring sufficient support for ISS through 2030.

The spacecraft are owned and operated by the vendor, and crew transportation is provided to NASA as a commercial service. Each mission sends up to four astronauts to the ISS. Operational flights occur approximately once every six months for missions that last for approximately six months. A spacecraft remains docked to the ISS during its mission, and missions usually overlap by at least a few days. Between the retirement of the Space Shuttle in 2011 and the first operational CCP mission in 2020, NASA relied on the Soyuz program to transport its astronauts to the ISS.

A Crew Dragon spacecraft is launched to space atop a Falcon 9 Block 5 launch vehicle and the capsule returns to Earth via splashdown in the ocean near Florida. The program's first operational mission, SpaceX Crew-1, launched on 16 November 2020. Boeing Starliner spacecraft will participate after its final test flight, launched atop an Atlas V N22 launch vehicle. Instead of a splashdown, a Starliner capsule will return on land with airbags at one of four designated sites in the western United States.

Development of the Commercial Crew Program began in 2011 as NASA shifted from internal development of crewed vehicles to perform ISS crew rotation to commercial industry development of transport to the ISS. A series of open competitions over the following two years saw successful bids from Boeing, Blue Origin, Sierra Nevada, and SpaceX to develop proposals for ISS crew transport vehicles. In 2014, NASA awarded separate fixed-price contracts to Boeing and SpaceX to develop their respective systems and to fly astronauts to the ISS. Each contract required four successful demonstrations to achieve human rating for the system: pad abort, uncrewed orbital test, launch abort, and crewed orbital test. Operational missions were initially planned to begin in 2017, with missions alternating between the two providers. Delays required NASA to purchase additional seats on Soyuz spacecraft up to Soyuz MS-17 until Crew Dragon missions commenced in 2020. Crew Dragon continues to handle all missions until Starliner becomes operational no earlier than 2026.

In 2004, the Aldridge Commission – established by President George W. Bush following the Space Shuttle Columbia disaster – called for crewed flights to the Moon with a Crew Exploration Vehicle in its final report. Following the NASA Authorization Act of 2005, the Constellation program was established, which envisioned a revised Crew Exploration Vehicle named Orion conducting crew rotation flights to the International Space Station (ISS) in addition to its lunar exploration goals. Orion superseded the Orbital Space Plane, which was specifically designed for ISS crew rotation. In 2009, the Augustine Commission appointed by President Barack Obama found that the program's funding and resources were insufficient to execute its goals without significant delays to its schedule and an increase of US$3 billion in funding, which prompted NASA to start considering alternatives to the program. The Constellation program was officially cancelled in 2010, with NASA repurposing Orion for exploration beyond Earth, and collaborating with commercial partners for ISS crew rotation and other crewed activities in low Earth orbit following the retirement of the Space Shuttle program in 2011. This arrangement would additionally end NASA's dependency on Roscosmos' Soyuz program to deliver its astronauts to the ISS.

The NASA Authorization Act of 2010 allocated US$1.3 billion for an expansion of the existing Commercial Crew Development (CCDev) program over three years. While the program's first round of competition in 2010 focused on funding development of various human spaceflight technologies in the private sector as part of the American Recovery and Reinvestment Act, its second round, CCDev 2, focused on proposals for spacecraft capable of shuttling astronauts to and from the ISS. The competition for CCDev 2 funding concluded in April 2011, with Blue Origin receiving US$22 million to develop its biconic nose cone capsule concept, SpaceX receiving US$75 million to develop a crewed version of their Dragon spacecraft and a human-rated Falcon 9 launch vehicle, the Sierra Nevada Corporation receiving US$80 million to develop the Dream Chaser, and Boeing receiving US$92.3 million to develop the CST-100 Starliner. SpaceX had previously been contracted by NASA to operate ISS resupply flights with their Dragon spacecraft, as part of NASA's Commercial Resupply Services. The program's third round, Commercial Crew integrated Capability (CCiCap), aimed to financially support the development of winning proposals over 21 months through to May 2014, in preparation for crewed missions to the ISS within five years. Despite winning awards in CCDev 1 and CCDev 2, Blue Origin decided against competing in CCiCap, opting instead to rely on private investment from their owner, Jeff Bezos, to continue development on crewed spaceflight. The competition for CCiCap funding ended in August 2012, with US$212.5 million allocated to Sierra Nevada's Dream Chaser, US$440 million allocated to SpaceX's Crew Dragon, and US$460 million allocated to Boeing's Starliner. While Alliant Techsystems's integrated Liberty launch vehicle and spacecraft was a finalist, it was rejected due to concerns about the lack of detail in Alliant Techsystems's proposal.

In December 2012, the three CCiCap winners were each given an additional USD$10 million in funding as the first of two series of "certification products contracts" (CPC) to allow for further testing, engineering standards, and design analysis to meet NASA's safety requirements for crewed spaceflight. The second CPC series manifested as Commercial Crew Transportation Capability (CCtCap), the final phase of the CCDev program, where NASA would certify an operator to run crewed flights to the ISS through an open competition. The window for proposal submissions was closed on 22 January 2014. Sierra Nevada announced a week later that a privately funded orbital test flight of a Dream Chaser spacecraft, using an Atlas V launch vehicle intended to be purchased by Sierra Nevada, was planned to occur on 1 November 2016. On 16 September 2014, CCtCap concluded with SpaceX's Crew Dragon and Boeing's Starliner being the sole winners, with SpaceX receiving US$2.6 billion contract and Boeing a US$4.2 billion contract. Sierra Nevada filed a protest with the Government Accountability Office (GAO) in response, citing "serious questions and inconsistencies in the source selection process." The United States Court of Federal Claims upheld a decision to allow development of the Crew Dragon and Starliner to proceed during the protest, citing concerns for crewed operations of the ISS in the event of a delay to the Commercial Crew Program. The GAO declined Sierra Nevada's protest in January 2015, stating that evidence gathered by the GAO discredited Sierra Nevada's claims against NASA; Sierra Nevada accepted the decision. The company laid off 90 staff members working on the Dream Chaser following the CCtCap result, and repurposed the spacecraft as a for-hire vehicle for commercial spaceflight. A cargo variant of the Dream Chaser would later be developed and selected by NASA to fly uncrewed resupply missions to the ISS under a Commercial Resupply Services 2 contract.

While the first flights of Commercial Crew Program were originally intended to be launched by the end of 2017, Boeing announced in May 2016 that their first crewed flight would be delayed to 2018 due to problems integrating with Starliner's Atlas V N22 launch vehicle. In December 2016, SpaceX announced their first crewed flights would also be delayed to 2018, following the loss of AMOS-6 in an accidental launch pad explosion of a Falcon 9, the Crew Dragon's launch vehicle. With no further flights in the Soyuz program for American astronauts past 2018, the GAO expressed concerns and recommended in February 2017 that NASA develop a plan for crew rotation in the event of further delays. Following the settlement of a lawsuit against Russian space manufacturer Energia over Sea Launch, Boeing received options for up to five seats on Soyuz flights, which NASA purchased from Boeing. NASA announced the astronauts chosen to pilot the Crew Dragon and Starliner vehicles in August 2018, and two months later penned the launch of demonstration missions for the Crew Dragon and Starliner for dates in 2019. The uncrewed SpaceX Demo-1 mission was launched on 2 March 2019, in which a Crew Dragon successfully docked with the ISS and returned to Earth six days after launch. The capsule used in the mission, however, was accidentally destroyed in a static fire test of its SuperDraco engines in April 2019, causing further delays to launch of future Crew Dragon flights. The Boeing Orbital Flight Test and Boeing Crew Flight Test, which had both been delayed due to a failed test of Starliner's abort system, were further pushed without explanation from dates in early-to-mid 2019 to late 2019.