Ruben Studdard -- Giant Tax Lien ... Again

It's never a good sign when a singer has had more tax liens than platinum albums -- but that's the situation former "American Idol" champ Ruben Studdard has found himself in ... TMZ has learned.

According to docs filed in Alabama, Studdard and his estranged wife Zuri owe $180,216.73 in federal taxes for the year 2009.

Studdard is no stranger to tax problems in Alabama -- he had a property lien filed against him back in 2008 to the tune of nearly $200,000.

Ruben filed for divorce from Zuri back in November.