Raisman grabbed a bronze medal earlier in the day on the balance beam when the judges revised her score upward after the competition, but there was no question about her final routine on the floor.
She received a score of 15.600 points for that exercise with Romania's Catalina Ponor finishing second at 15.200.
It was Ponor who had appeared to have won the balance beam bronze before Raisman's score was boosted by a tenth of a point. That created a tie between the Romanian and the American, but because Raisman's execution score was higher she received the bronze.
All-around gold medalist Gabby Douglas of the United States participated in both the beam and floor exercise final Tuesday, but did not medal in either.
The American women won five gymnastics medals in six events during the Olympics, three of them gold.
Chinese gymnasts Deng Linlin and Sui Lu finished in front of Raisman on the balance beam and the other individual apparatus gold medals awarded Tuesday went to China's Feng Zhe on the parallel bars and to Epke Zonderland of the Netherlands on the high bar.
China's men had a poor qualifying effort on the opening day of the Olympics, but wound up winning the team gold medal and four other medals in the individual events.
The United States men led qualifying and yet they claimed only one Olympic medal, a bronze by Danell Leyva in the all-around.
Those in the women's floor exercise won by Raisman final included defending Olympic champion Sandra Raluca Izbasa of Romania, current world champion Kseniia Afanaseva of Russia and Ponor, who won the event at the 2004 Olympics.
Aliya Mustafina of Russia, who won a gold medal on Monday in the uneven bars, was the bronze medalist in floor exercise on Wednesday.