Former soap actor Scott Evans -- younger brother of Chris Evans -- got busted for attempting to buy drugs off an undercover cop ... TMZ has learned.

Law enforcement sources tell us Evans was arrested Tuesday evening when he tried to score narcotics from an LAPD officer posing as a dealer.

Evans -- who played Officer Oliver Fish on "One Life to Live" a few years back -- was booked at 6:15PM for attempt to possess a controlled substance ... a felony. He bailed out around 12:30AM.

It's unclear what drug Evans was trying to buy.

Evans reacted to the bust with a Facebook post early this morning: "Most ridiculous night of my life in LA so far. At least I made it a year before getting arrested."