The 29-year-old Vollering also won the 2023 title and claimed her second one by timing the nine-day slog to perfection in winning the final two stages, which both ended in Nice.
"It's a dream to live this life," Vollering said after hoisting her bike above her head at the finish line.
"I knew my level and even when Kasia won on Mont Ventoux I was still confident," added Vollering, who has also finished second on this race twice in its five editions.
Niewiadoma took the yellow jersey on Friday's climb of Mont Ventoux, setting up a pulsating finale.
Vollering then edged into the overall lead on Saturday, attacking late on a small hill and edging into an eight-second overall lead on Niewiadoma.
A furious Niewiadoma, however, complained she had been blocked by a teammate of Vollering's at a crucial moment, but no action was taken.
Vollering said her team had been both honest and humble after Sunday's race adding that her teammate Celia Gery, the alleged blocker, had received death threats on social media.
"Celia had a really bad night. I'm sure deep down that Kasia knows these kind of things can happen in a race," Vollering said.
The scene was set for a tense final stage that was contested over four climbs of the Col d'Eze just outside Nice, where Vollering waited until the fourth ascent to pounce.
Attacking near the summit, she took a 20sec lead and thundered down towards Nice never looking back.
Italy's Elisa Longo Borghini came third overall after Swiss rider Marlen Reusser fell on a descent and lost a two-minute cushion on the podium.
Dutch sprinter Lorena Wiebes won the green sprint points jersey mainly thanks to her victories on the two opening stages.
Also from the Netherlands Puck Pieterse secured the polka dot climb points jersey ahead of the final stage.
The best young rider jersey was won by Germany's Antonia Niedermaier.