Donald Trump to Miss PA -- You've Got 24 Hours to Apologize ... OR ELSE!!!

Former Miss Pennsylvania Sheena Monnin has 24 hours to retract her claim that the Miss USA pageant is rigged -- or Donald Trump will slap her with a nasty defamation lawsuit ... this according to Donald's lawyer.

Trump's Special Counsel Michael Cohen called in to TMZ Live moments ago to say Donald believes everyone deserves a second chance -- which is why he's giving Monnin one day to withdraw ... in writing ... "her utterly baseless claim that the Miss USA pageant results were predetermined."

If Monnin fails to comply, Cohen says the 27-year-old can bet her ass Trump will sue ... seeking "massive damages."