The sad news was announced by her co-star Justin Guarini on Instagram on Saturday.
He wrote: “@realnikkimckibbin was a fiery, funny woman who could sing the hell out of a rock song with the same kind of ease and control that she lovingly used to cut you off with her southerner spirit.
“Even in our twenties when we were American Idol together I could tell that she had had a difficult life, and that not many people had been nice to her along the way … but I will never forget the day her idol, Stevie Nicks, sent her flowers with a card that said “You are the Gypsy I was…”.
“Nikki was in a new cloud and the joy and excitement emanating from her was infectious. After the kind of emotional blows she took at the hands of the masses, she deserved to feel good about herself, even for a little while, ”he writes.
“Rest well, ‘Gypsy’… and thank you for the laughter, the merciless teasing, the strength, the vulnerability, the love and the friendship that you have shown me during our time together in the spotlight.
TMZ reports that sources told them Nikki died of a brain aneurysm.
The US outlet claims the singer was in hospital on life support and at 3 a.m. she was pronounced dead.
Nikki finished third in the 2002 American Idol series.
Justin Guarini was second and Kelly Clarkson was crowned the winner.
Nikki was open about her problems and had substance abuse issues.
She appeared in 2008 on Dr. Drew’s Celebrity Rehab and went on to be on Sober House, a Celebrity Rehab spin-off.
Nikki had been married to Craig Sadler since 2007.