It was Japan's fourth wrestling gold of the London Games but a first for the Japanese men's team.
Yonemitsu and Kumar won bronze medals at the Beijing Olympics in 2008.
"I was lucky. It is impossible to do it just with my strength," Yonemitsu said. "It's because of a combination of luck, good condition, good performance and everything."
India's Kumar took India's first silver in Olympic wrestling and said a stomach illness took the edge off his performance.
"Without the stomach infection, I could have tried harder," said Kumar, who was India's flag-bearer at the opening ceremony.
Japan's dominant women wrestlers won three golds and the men also picked up two bronzes to finish second in the final medal table, just behind Russia.