The company will rebrand as Virgin Hyperloop One and Sir Richard will join the Los Angeles-based firm's board as part of the investment, worth an undisclosed amount.
The Hyperloop will work by loading passengers and cargo into a pod and accelerating gradually via electric propulsion through a low-pressure tube.
In 2013, Musk pitched an idea for entrepreneurs to create a massive vacuum tube that could one day transport someone from Los Angeles to NY in 45 minutes.
Hyperloop One is developing a system theorized by entrepreneur Elon Musk for rail transport at near-supersonic speeds, and in August announced that a test of full hyperloop system at its private facility in the desert near Las Vegas was a success, hitting record speeds. He described the hyperloop concept as "a cross between a Concorde, a railgun and an air hockey table". The terms of the deal weren't disclosed, but the investment was significant enough that Hyperloop One will now be called Virgin Hyperloop.
The startup had raised more than $200 million prior to Tuesday's announcement.
From now on the company, which is currently making rapid progress on its test track in Nevada, will be rebranded as Virgin Hyperloop One.
In a blog post on Thursday, Branson said: "We're incredibly excited about the technology behind Virgin Hyperloop One and the way it could transform passengers' lives".
TechCrunch notes that included in the re-branding, Virgin Hyperloop One will now be associated with Virgin Group's other projects. Branson attended and said he saw "firsthand the exciting technology being tested". The company was embroiled in a messy lawsuit past year between its two founders.
In July, Musk dropped the H-word in a tweet about the project: "Just received verbal govt approval for The Boring Company to build an underground NY-Phil-Balt-DC Hyperloop".