Besik Kudukhov of Russia and Bulgaria's Radoslav Velikov, both former world title holders, won bronze medals.
Cejudo, a 21-year-old from Colorado Springs taking part in his first Olympics, had tears in his eyes as he wrapped himself in the Stars and Stripes, following a winning campaign he attributes to his "American way" of non-stop wrestling.
The coach of Matsunaga, 28, had disputed a point in the first period but judges reviewed the video tape and ruled in favour of the American and let it stand.
"He's testament to the fighting spirit of America," coach Mike Duroe said. "This means so much to him. Gold medals are the American dream."
Cejudo, with an eye coming up black, said: "My life has changed already. Watching the flag go up on that podium feels real good."
The reigning US national champion in his weight class, marked himself out at college by his iron dedication, hitting the weights or running a quick three or four miles between practice sessions.
Cejudo was born in Los Angeles to then illegal immigrants from Mexico City who met in the United States.
Matsunaga beat favourites Dilshod Masurov of Uzbekistan, the 2003 and 2005 world champion, and Russian champion Besik Kudukhov on his way to the final match with Cejudo.
"He was exhausted after beating two world champions," said former champion wrestler Tomiaki Fukuda, head of the Japanese Olympic team. "His energy was gone before he got to the final."