$US22 million tax

Boxer Floyd Mayweather owes US$22 million tax

A notice of a federal tax lien filed against the undefeated fighter nicknamed "Money" shows the balance as unpaid as of March 6.

The document was filed with county officials in Las Vegas in April.

Mayweather's tax troubles come as he gets ready to come out of retirement to fight against Irish MMA star Conor McGregor in August.

The lien for 2015 is just the latest in a series of tax liens filed by the IRS against Mayweather over the past decade.

Mayweather's tax lawyer did not immediately return a request seeking comment on Tuesday.