IRS to Kevin Federline -- You Owe Us $57,000 ... Pay It Now

Uncle Sam has a bone to pick with Kevin Federline -- and no, it's not for releasing "PopoZao" -- it's because the IRS says K-Fed still owes back taxes ... to the tune of $57,615.39.

A federal tax lien was filed against Britney Spears' baby daddy on February 14th ... claiming K-Fed has outstanding income tax bills for the years 2009 ($30,517.16) and 2010 ($27,098.23).

In case you're wondering how K-Fed made money in those years -- besides those hefty spousal support payments -- in 2009 he had a bit part in American Pie: The Book of Love (straight-to-video) and in 2010 he starred in "Celebrity Fit Club" on VH1.

Probably not making him money ... PopoZao. Memba' that?