Trey Songz IRS Says He Went 'Bottoms Up' On Taxes

Trey Songz skipped out on a major tab with Uncle Sam for about $750k ... according to the IRS which, as it always does, has come calling for the dough.
Trey got hit with the federal tax lien last month in Miami -- letting him know he owes $748,870.08 on his income from 2013. That happens to be the same year he plunked down $35,000 a month to live in a Hollywood Hills mansion. Priorities?

Unclear how or why Trey fell short ... we've got a call into his rep, but no word back yet.
It ain't chump change, but we're guessing another hit song and world tour can fix it all real quick.