The billionaire announced the deal Friday at the hotel ... which he plans to rebrand as The Virgin Hotel Las Vegas. The hotel will remain open as it undergoes renovations ... it's expected to be completed in 2019.

The deal included other partners ... including the LA-based Juniper Capital Partners. It's unclear how much Branson dropped to add a second hotel to his portfolio. The only other Virgin hotel is in Chicago.

Here's hoping Branson knows what he's gotten into ... the Hard Rock Hotel's seen a lot in its 23 years of existence. Especially some brawls.