President Trump says Rep. Neal Dunn had a terminal diagnosis that had him dying in June ... and POTUS claims he saved the guy.

Trump publicly revealed the grim outlook the GOP Congressman from Florida was facing in a press conference Monday on Capitol Hill.

GOP leaders were talking about their slim voting edge in the House when Trump brought up one Congressman in his party who was very ill ... Dunn.

While Speaker Mike Johnson was keeping the diagnosis close to the vest, Trump said Dunn had been told he would be dead by June with a heart issue ... and that would have been bad for passing legislation along party lines.

Trump got Dunn hooked up with some top doctors at Walter Reed Medical Hospital ... and Johnson says the lawmaker has a new lease on life and feels 30 years younger.

Check out the clip ... Johnson clearly doesn't want to put this all out in public, but Trump just couldn't keep this secret.