Dr. Luke Sues -- Kesha's Lawyer Screwed Me When He Called Me a Rapist

Dr. Luke is firing back at Kesha's attorney with a lawsuit ... after he accused the high-powered producer of raping Lady Gaga -- something Luke calls an "outrageous" lie.

TMZ broke the story ... after Lady Gaga told Howard Stern last week that someone in the music industry had raped her -- Mark Geragos went on Twitter and asked, "Guess who the rapist was?"

When someone guessed Dr. Luke ... Geragos responded, "#bingo." When we contacted Geragos he doubled down ... telling us he said it "because it's true." Of course, Geragos is representing Kesha in her sexual assault lawsuit against Luke.

In his defamation lawsuit, filed today, Luke calls Geragos' comment "maliciously false." Gaga backed that up last week, telling TMZ it's absolutely untrue that Luke was the mystery rapist.