Raisman added gold to the bronze she had earned for the balance beam 90 minutes earlier and the team gold she helped the United States to win a week ago.
Romanian Catalina Ponor, demoted to fourth on the beam after the Americans won an appeal on Raisman's score, had the compensation of floor silver while Russia's Aliya Mustafina took bronze on a tiebreak ahead of Italian Vanessa Ferrari.
American Jordyn Wieber, who missed the chance to qualify for the all-around final here despite being world champion, went home with no individual medals, finishing seventh on the floor after stepping out of bounds twice.
The only woman behind her was Izbasa who crashed to her knees as she tried to land her final tumbling pass and incurred a penalty of 0.3 points.
World champion Kseniia Afanaseva of Russia, who opened the final, finished sixth after being penalised for two oversteps.
The 18-year-old Raisman, who turned out to be the U.S. team's most successful woman at the Games, produced a top-quality routine, accompanied by music from a Hebrew folk song that had the crowd at North Greenwich Arena clapping along.
When she finished off with a perfectly-landed backflip and double-pike somersault combination she ran off to receive a big hug from her coach. A score of 15.600 earned her another bear hug and she waited for the remaining five finalists to compete.
Raisman said her beam bronze had taken the pressure off her ahead of the floor event.
"I had nothing to lose because I had already achieved one of my goals and it was going to be my last memory for London so I was going to enjoy it," she told a news conference.
"When I was in my floor routine I was really calm and I did hear everyone who was cheering and I was trying to have fun and stay in the moment."
Ponor, the 2004 Olympic floor champion, came out of retirement to help Romania in London and the 24-year-old said she would give up competition again with no regrets after helping the team to win bronze here.
"I have already been an Olympic champion," she told reporters. "I will quit gymnastics with my chin up."
Mustafina, who already had asymmetric bars gold, team silver and all-around bronze in her pocket, and 2006 world all-around champion Ferrari both finished with 14.900 points, 0.3 behind Ponor.
The tiebreak rule came into play and separated them on their execution score - one of two elements that make up the final mark. Mustafina's was 9.000, Ferrari had 8.700.
"I didn't expect a medal when I did my floor exercise but I liked how the competition ended," Mustafina told reporters. "I'm very happy and very tired."