'Real Housewives of Atlanta' Star Kim Zolciak's Ex Lee 'Big Poppa' Najjar Dead

Lee Najjar -- best known to "The Real Housewives of Atlanta" fans as Kim Zolciak’s elusive "Big Poppa" sugar daddy -- has died at 68.

The wealthy Atlanta real estate investor passed away Saturday, April 18 -- tragic news confirmed by his daughter Katelin via a slate of IG Story pictures of them together and an emotional tribute. Lee's cause of death is unknown.

The Miami-Dade Medical Examiner also confirmed his death to TMZ.

Najjar became a notorious off-camera figure during the early seasons of 'RHOA' -- as the mystery man bankrolling Zolciak’s luxe lifestyle -- all while he was still reportedly married to another woman.

He and his wife shared four children together -- Jared, Jeremy, Katelin and Jamen Najjar.

"Big Poppa" was never fully shown on camera, but his presence loomed large ... fueling one of the show’s juiciest early storylines and helping cement 'Housewives' as must-watch TV.

Kim said back in 2009, when their romance ended, "He will always be the love of my life, but it’s time for me to move on" ... and it wasn’t long after that his identity became public.

He was 68.