"We are so excited to be part of Nasa as our home and laboratory in space transitions into being accessible to expanded commercial and marketing opportunities, as well as to private astronauts", said astronaut Christina Koch, in a Nasa tweet.
Visitors would be able to stay up to 30 days to "perform duties that fall into the approved commercial and marketing activities outlined in the directive".
Up until now, NASA had not allowed ISS to be used for commercial purposes.
There will be up to two short private astronaut missions per year, said Robyn Gatens, the deputy director of the ISS.
The announcement is a significant change for the agency, which has had a long-standing prohibition against allowing tourists and commercial interests on the station, which has cost taxpayers about $100 billion over its lifespan. The two companies are set to ferry astronauts to the ISS from USA soil for the first time in almost a decade.
The private astronaut aspect of the initiative is particularly intriguing.
But, a trip to space won't come cheap - with life support systems and all necessary supplies considered, it will cost an eye-watering $35,000 per night.
NASA administrator Jim Bridenstine suggested previous year the agency would consider being open to branding deals, so the marketing activities aspect of today's announcement doesn't come as a surprise.
As noted during NASA's press conference, a key factor in commercializing space will be reducing associated costs.
'Transitioning to this new model of business is an important step to enable NASA to move full speed ahead toward our goal of landing the first woman and the next an on the moon. "And so, (private astronauts would) have to contract with them and whatever prices Boeing and SpaceX set is on them".
"We're going to re-evaluate the pricing every six months", Jeff DeWit, NASA's chief financial officer, said of the agency's charges.
"But it won't come with any Hilton or Marriott points", DeWit deadpanned.
Gerstenmaier said NASA will expect Boeing and SpaceX to handle all the arrangements for visits by commercial astronauts. "What we're hearing is a lot of excitement in the commercial sector for this".
Nasa had previously banned any commercial use of the space station and prohibited astronaut's from taking part in for-profit research.
"I's hard to project what's going to come back", DeWit said.