Wes Scantlin -- Busted For DUI ... Again

Wes Scantlin just can't seem to figure out the whole obey the law thing ... the Puddle of Mudd singer was busted for DUI for the second time in under a month.

Law enforcement sources tell us Wes was pulled over for speeding early Wednesday morning in South Dakota. We're told cops determined he was driving under the influence, but it's unclear if he failed a breathalyzer test or field sobriety testing ... or both.

Ever the overachiever, Scantlin was also charged with pot possession.

TMZ broke the story ... Wes was busted in July for DWI in Minnesota after he blew 4x the legal limit.

Scantlin is set to perform in SD tonight after getting released on $500 bail.