It's pretty much the last thing Iggy Azalea needs after the week she's had with fiancé Nick Young ... but the IRS says she's run up a huge tab with them, and it's time to pay up.
Uncle Sam just slapped the rapper with a huge tax lien -- $391,056.55 in unpaid income taxes from 2014. Remember, that was a huge year for Iggy, since her debut album "The New Classic" dropped ... and became a huge hit.
Great timing ... since she's not dealing with allegations her guy cheated or anything. Oh wait.
You can always count on the taxman ... to make a REALLY bad situation, way badder.