SAN FRANCISCO — SpaceX, the rocket maker founded by billionaire Elon Musk, has raised up to $350 million in new financing and is now valued at around $21 billion, making it one of the most valuable privately held companies in the world.

SpaceX’s new financing was disclosed in public filings that were obtained by Equidate, a marketplace for private company stock. SpaceX did not have an immediate comment.

With the latest funding round, SpaceX joins an elite club of seven venture-backed companies valued at $20 billion or more around the world, according to research firm CB Insights. Investors have poured money into the companies, many of which operate capital intensive businesses such as Uber and Airbnb, even as smaller start-ups have gone public and have seen their valuations waver.

Snap, the messaging and entertainment company, went public in March at a market capitalization of $24 billion, which has since fallen to about $16.5 billion. Meal delivery company Blue Apron went public last month at a market capitalization of $1.9 billion, which was less than its private market valuation; that has since dropped to around $1.3 billion.

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Still, investors have long believed that a handful of companies have the potential to become globally dominant businesses. Five companies based in the United States that are valued at more than $20 billion — including Uber, Airbnb and WeWork — have upended established industries like transportation and real estate. Palantir, the analytics company that is in the $20 billion-plus valuation club, is vying to become a major government contractor, as is SpaceX.

The two other closely held companies valued at more than $20 billion are both in China — ride-hailing company Didi Chuxing and consumer electronics maker Xiaomi.

SpaceX’s latest funding nearly doubles the valuation of the company, which was pegged at around $11 billion when it raised $1 billion from Fidelity and Google in 2015. Previous investors in SpaceX include venture capital firms Founders Fund and DFJ.

SpaceX is best known for Mr. Musk’s goal of colonizing Mars, but it is also a key player in the business of sending commercial satellites into space.

Last September, the company suffered a major setback when, during fueling for an engine test, one of its Falcon 9 rockets caught fire on the launchpad and exploded, destroying a $200 million satellite. SpaceX resumed launching in January, and, after years of failing to meet its promises, the company has finally achieved a steady cadence, with 10 launches already this year. By the end of the year, SpaceX hopes to finally get its bigger, years-delayed Falcon Heavy rocket off the ground.

In March, the company launched a partly used, less expensive rocket, with the aim of bringing down the cost of sending supplies into space. Replicating such flights with reusable rockets is key for Mr. Musk’s vision of sending people to Mars.

“It means you can fly and refly an orbital class booster, which is the most expensive part of the rocket,” Mr. Musk said at the time.

Mr. Musk faces potential competition in the rocket space from another billionaire. Blue Origin, a rocket company founded by Jeff Bezos, the chief executive of Amazon, also aims to send tourists and supplies into space. Mr. Bezos said in April that he’s selling about $1 billion of Amazon stock every year to fund the company.