Venus and Serena Williams, meanwhile, at least partly made up for their disappointing exit from singles action by capturing the doubles title.
Nadal, poised to take over the No. 1 ranking after Roger Federer had dominated the sport for a half decade, defeated Fernando Gonzalez of Chile in the gold medal match 6-3, 7-6 (7-2), 6-3.
The Spanish superstar added the gold medal to his Wimbledon title, which he earned in an epic, five-set victory over Federer.
Federer had been trying for his first singles gold in three Olympic appearances, but lost in the quarterfinals. He came back Saturday to win the doubles title with fellow Swiss Stanislas Wawrinka.
Elena Dementieva won the women's singles gold medal Sunday by rallying for a 3-6, 7-5, 6-3 win over Dinara Safina.
Vera Zvonareva made it a Russian sweep with a 6-0, 7-5 victory over China's Li Na in the bronze medal match.
It was the first time the same country has swept an Olympic tennis event since the sport was put back in the Olympics 20 years ago.
The Williams sisters made short work of Anabel Garrigues and Virginia Ruano Pascual of Spain in the doubles final 6-2, 6-0.