Cai and Fu, the favourites as four-time world champions, defeated Mathias Boe and Carsten Mogensen of Denmark 21-16, 21-15.
China missed out on a sweep in their Beijing Games when Cai and Fu lost their final, but they became the first Chinese pair to win the doubles gold.
"Of course, we were very nervous but we thought we played very well," Fu said. "We're very proud of all five medals."
Denmark's Queen Margrethe and the minister of sport came to watch Boe and Mogensen, but the duo appeared to have given their best in their semifinal win over the top-ranked South Koreans.
They were still delighted to help the team win two badminton medals in a single Olympics for the first time.
"China are the No. 1 badminton nation," Mogensen said. "It is a little bit annoying that they are as good as they are, but it is a well deserved win for China."
South Korea won't take home a gold or silver from Olympic badminton for the first time, but Chung Jae-sung and Lee Yong-dae ensured they won't leave London without a medal by winning the bronze.
Upset in the semifinals by the Danes, Chung and Lee rebounded to beat Koo Kien Keat and Tan Boon Heong of Malaysia 23-21, 21-10.
It was Lee's second Olympic medal, after he won the mixed doubles gold in Beijing.
Chung, his partner for seven years, announced he was retiring, and said he was glad they overcame his shoulder and back problems this year and made up for their first-round loss at the Beijing Games.
"This Olympics has a special meaning for me. I didn't want to fail twice. I wanted to succeed this time," Chung said. "Although not able to get a good performance in qualifiers, we regarded this bronze medal competition as the final for us, and that's why we were able to get the medal."
Lee, regarded as the best men's doubles player in the game, said he will miss Chung.
"It has been my great fortune that I was able to work with big brother Chung," he said. "I met him in lower sixth form (12th grade) when I was not as good as I am today. I learned a lot from him."
Koo and Tan, former No. 1s and two-time All England Open champions, let slip a 19-13 lead in the first game and two game points. They never recovered.