The last Japanese wrestler to claim gold before Yonemitsu was flyweight Mitsuru Sato at the Seoul Games in 1988.
Yonemitsu, the runner-up at the world championships in 2011, beat Kumar 1-0, 3-1 in the first gold-medal match featuring an Indian wrestler.
The win was Japan's fourth gold of the London Games. Three Japanese women won gold medals earlier this week.
"It's unbelievable that I really did it. I think that every day's training paid off," Yonemitsu said. "If you did your best just on the day, it would not work. You need to train step by step.
"I was lucky. It is impossible to do it just with my strength. It's because of combination of luck, good condition, good performance and everything."