The victory added to Serena's singles gold she won on Centre Court at Wimbledon a day earlier against Maria Sharapova, becoming just the second woman ever to earn a Golden Slam.
The American sisters beat Andrea Hlavackova and Lucie Hradecka of the Czech Republic 6-4, 6-4 under the roof on a rainy afternoon at the All England Club.
Serena became tennis' first double gold medalist at an Olympics since Venus won singles and doubles at the 2000 Sydney Games. The sisters also won the doubles gold at the 2008 Beijing Olympics. Now, they each have a record four Olympic tennis gold medals, and the sisters didn't drop a set through their five matches at the London Games.
In men's tennis, Andy Murray of Britain beat Roger Federer 6-2, 6-1, 6-4 for Olympic gold. The victory marked a career breakthrough for Murray. He has lost all four of his Grand Slam finals, three against Federer, including Wimbledon a month ago.
In track and field, Usain Bolt won gold in 100 meters with the second-fastest time in history: 9.63 seconds. Undeterred from his goal of becoming a "living legend," Bolt returns to Olympic Stadium on Monday to receive his medal. A day later, he'll begin running in his favorite race, the 200. American Wallace Spearmon should be the other prime contender in that race.
The Olympics' year of the woman hit another milestone when women's boxing made its debut in the games Sunday. The tournament began with 12 entertaining bouts featuring uppercuts, haymakers and footwork that all measure up nicely to the men's amateur sport.
Russia's Elena Savelyeva won the first fight with a busy jab and strong combinations. U.S. lightweight Queen Underwood lost a close bout with Britain's Natasha Jonas.
McKayla Maroney was all set to add the vault title to her team gymnastics gold with the U.S. when she made a costly mistake. She appeared to land her second vault on the backs of her heels. Her feet slid out from under her, and she plopped on the mat, a look of shock crossing her face.
In women's basketball, the U.S. matched their Olympic scoring record in a 114-66 rout of China.
The big story Saturday focused on Michael Phelps, who closed out his storied career with another gold. The most decorated Olympian of all time and his teammates won the 4x100 medley relay. It was Phelps' 1 8th gold.
Meantime, Irvine's Dotsie Bausch won silver in women's team pursuit cycling. Bausch, a former model, overcame a severe eating disorder to become an athlete.
Two-time gold medalists Kerri Walsh Jennings and Misty May-Treanor beat the Netherlands to advance to Sunday's quarterfinals in women's beach volleyball.
Also, the U.S. men's basketball team came from behind in the 4th to defeat Lithuania 99-94 in preliminary round play. The U.S., unless it loses by at least 17 points, will open the quarterfinals Wednesday against Australia, the fourth-place team from Group B.
The Associated Press contributed to this report.