Young Thug's off the hook for a felony drug charge, thanks to his lawyers arguing cops overstepped boundaries with an illegal search warrant.

You'll recall the rapper's crib was raided by cops in Sandy Springs, GA in July 2015 ... turning up weapons and drugs. Thug was charged with felony cocaine possession, felony marijuana possession and 3 counts of felony gun possession.

Since then, the D.A. had dropped all but the marijuana charges. YT's lawyers argued that should be kicked out too, because cops conducted the search without a proper warrant. The judge agreed ... giving YT total victory.

We reached out to the D.A. for comment, but so far no word back.