Spain's Nadal, 36, won 6-3 6-3 6-0 against Norwegian eighth seed Ruud to also extend his record number of Grand Slam men's singles titles to 22.
He moves two ahead of his great rivals Roger Federer and Novak Djokovic.
Nadal, who lost to Djokovic in the semi-finals last year, has won 112 of his 115 matches on the Paris clay.
After also winning the Australian Open in January, Nadal has claimed back-to-back major titles for the first time since 2010 - when he won the French Open, Wimbledon and US Open in a row.
Two days after his 36th birthday, Nadal becomes the oldest French Open men's singles champion and surpasses fellow Spaniard Andres Gimeno, who won aged 34 in 1972.
The left-hander dropped his racquet to the court in amazement after taking his second match point to win in two hours and 18 minutes.
Nadal's achievements are even more impressive in the context of the physical issues that have hampered him in recent times.
At the end of last year he thought he would have to retire because of a chronic foot problem and was troubled by the issue at the Italian Open earlier this month.
A stress fracture of the rib also caused him to miss two months of the season shortly after his 21st major win at the Australian Open.
"It takes a lot of energy to keep going. I want to say 'merci, merci beaucoup'," Nadal said after lifting the Coupe des Mousquetaires.
"I don't know what will happen in the future but I will keep fighting to keep going."