Olympics special site; Photo gallery
The 14-time Grand Slam winner was chosen for the honor at the London 2012 Games but missed out because of injury.
Moments before he entered Rio's Maracana stadium, the 30-year-old posted a photo of himself on Instagram with a beaming smile, reports Xinhua.
"Very happy to be here, really glad," he was quoted as saying by Rio2016.com. "It's something for every sportsman to be proud of, and of course, especially for me after not being in London," Nadal said on Friday.
Earlier this week Nadal, who won gold in the singles at the Beijing 2008 Games, confirmed he would be fit to play in Rio after missing the past two months with a wrist injury.