She crushed Russian Maria Sharapova in the championship match 6-0, 6-1, adding Olympic gold to her Grand Slam of professional tennis.
Victoria Azarenka of Belarus won the women's singles bronze medal with a 6-3, 6-4 decision over Maria Kirilenko of Russia in the third-place match.
The Bryans, bronze medal winners in 2008, defeated the French duo 6-4, 7-6 (7-2). Another French pairing, Julien Benneteau and Richard Gasquet, won the bronze medal match over David Ferrer and Feliciano Lopez of Spain.
Williams, who won the Wimbledon title last month, returned to the courts of the All England Club for the Olympics and played six singles matches in the tournament.
She did not come close to losing a set.
No opponent won more than three games in any of the 12 sets she played. She easily dispatched Jelena Jankovic of Serbia, Urszula Radwanska of Poland, Vera Zvonareva of Russia, Caroline Wozniacki of Denmark and Azarenka, the top seed, to reach the final.
She needed just 63 minutes to defeat Sharapova, who also was attempting to win the Golden Slam after completing the Grand Slam earlier this year with a win at the French Open.
Steffi Graf also won all four major championships as well as an Olympic singles gold medal. Roger Federer of Switzerland will try to do the same thing on Sunday in the men's final against Britain's Andy Murray.
Saturday's meeting was the 11th between Williams and Sharapova and Williams now holds a 9-2 advantage. Sharapova's only two victories in the series came in 2004.
Williams served 10 aces to one for Sharapova and hit 24 winners to just six for her opponent.
The singles gold medal was added to five wins at the Australian Open, one at the French Open, five at Wimbledon and three at the U.S. Open. She teamed with her sister Venus to win the doubles gold medal at both the Sydney and Beijing Olympics and they will have a chance to win a third thanks to a 7-5, 6-4 Saturday semifinal win over Russians Nadia Petrova and Marina Kirilenko.
The Williams sisters will face Andrea Hlavackova and Lucie Hradecka of the Czech Republic in the gold medal match.