France's 16-year-old Julia Pereira de Sousa Mabileau won silver, while Sochi gold medallist Eva Samkova of the Czech Republic claimed bronze.
Moioli, the current World Cup leader, just managed to get her board in front of the other five riders in a thrilling conclusion to clinch the gold by centimetres.
American Lindsey Jacobellis, in her fourth Olympics and chasing her first gold medal, could only manage fourth.
The format saw the top three riders from each quarter-final and the then semi-finals, with six competitors eventually contesting the final.
Sochi champion Samkova finished first in seeding and looked a strong candidate to defend her title. Her compatriot Vendula Hopjakova was less fortunate, as she was stretched off with a knee injury during seeding.
Both Samkova and Jacobellis got through to the final unscathed from their semi-final, as did Moioli and Sochi bronze medallist Chloe Trespeuch of France, meaning all the high-profile riders went into a highly-anticipated final.
The final lived up to its billing with all the riders contesting for space and the lead changing several times.
Jacobellis lead for much of the run, but it was Moioli and surprise package Pereira de Sousa Mabileau who came through to cross the finish line first.
Samkova held off 32-year-old Jacobellis for third.
Moioli said afterwards that this was the best day of her life.
"I want to thank everyone who has helped me on this long journey," said the 22-year-old.
"I cannot wait to see my medal. I am so happy and so proud of myself."
Athletes do not receive their medals immediately at Pyeongchang 2018, instead waiting for a ceremony at the Games' medal plaza.
"I felt the sensation that I was at the Olympics but I tried to stay focussed on the race and not everything else."
"Once I got ahead I had to stay solid and keep the girls behind me and I did that."
At 32, Jacobellis is twice the age of silver medallist Pereira de Sousa Mabileau and, despite a fourth Olympics without that elusive gold medal, the American is pleased she can still compete at this level.
"Fourth place is fourth place at the Olympics," said the Turin 2006 silver medallist.
"That I am still in the mix with these girls that are significantly younger than me is not something to shake my head at."
Jacobellis said she was still undecided if she would come back in four years time to have a fifth crack at clinching an Olympic gold.
(Editing by Greg Stutchbury/Amlan Chakraborty)
