With its shuttles about to retire, the agency has offered $270m (£166m) of funds to four firms to help them mature designs for new orbiting vehicles.
Blue Origin, Boeing, Sierra Nevada Corp and SpaceX hope to sell astronaut "taxi" services to Nasa by mid-decade.
Until then, US crews will have go to the space station on Russian rockets.
"The next American-flagged vehicle to carry our astronauts into space is going to be a US commercial provider," said Ed Mango, Nasa's Commercial Crew Programme manager.
"The partnerships Nasa is forming with industry will support the development of multiple American systems capable of providing future access to low-Earth orbit."
The winning companies have a range of concepts under development.
SpaceX, which has garnered much publicity recently, is perhaps the most advanced in its plans. It has already flown a rocket called Falcon 9 and a capsule called Dragon. It is being offered $75m over the next year if it meets certain milestones in advancing Dragon's crew-carrying capabilities.
The long-established Boeing company stands to win the largest award depending on developments. It has a capsule design called CTS-100 which could transport up to seven astronauts to the space station. The $92.3m Nasa support will help Boeing get the vehicle through to its preliminary design review.
Sierra Nevada Corporation has already received considerable financial support in Nasa's Commercial Crew Development (CCDev) effort, and is in line to get a further £80m in the latest round of funding. It is developing a shuttle-like vehicle that would launch atop a rocket.Boeing's Crew Space Transportation (CTS) 100 craft
The fourth recipient, Blue Origin, is a company set up by Amazon.com founder Jeff Bezos. Blue Origin has kept much of its space development activity secret, but it has requested funds from Nasa to help it mature systems for a cone-shaped crew vehicle. It has been awarded up to £22m.
Perhaps just as interesting as the companies that have won awards are the companies that have missed out.
These include ATK which makes the solid rocket boosters (SRBs) that lift the space shuttle off the ground. ATK wants to marry an evolution of these SRBs with the main core stage of Europe's Ariane 5 rocket. The concept, known as Liberty, would be used to launch other companies' capsules and spaceplanes.
ATK will now have to secure funds elsewhere if it wants to carry the Liberty idea forward.
Also missing out on CCDev money is United Launch Alliance (ULA). This is the company that operates Atlas and Delta rockets for the US Air Force and for Nasa.
These vehicles frequently orbit satellites, but ULA believes the rockets could be modified to launch humans also.
Sierra Nevada, Boeing and Blue Origin had all talked about using an Atlas 5 to loft their proposed crew ships.
Where Monday's announcement from Nasa leaves ULA's plans is uncertain. Again, it will need to use its own funds or find a partner if it wishes to continue with the project to man-rate the Atlas and Delta rockets.
Nasa is keen that the next era of human spaceflight include a strong commercial element. It plans to substantially increase its seed funding in 2012.
The philosophy is not shared by many in the US Congress who would prefer Nasa to lead the development of a shuttle successor along traditional procurement lines.ATK must seek alternative funds to continue developing its Liberty rocket idea Jonathan.Amos-INTERNET@bbc.co.uk
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