The packed arena turned into a roaring sea of red and white Japanese flags after seven-time world champion Icho swept aside China's Jing Ruixue in the final of the second heaviest 63kg class in freestyle wrestling.
Her weeping compatriot Obara held her head in her hands and fell to her knees after coming from behind to beat Azerbaijan's Mariya Stadnyk in Wednesday's other final, the 48kg lightest weight division.
Japan have ruled the mat since women first began competing in Olympic wrestling in Athens in 2004, winning six out of a possible 10 golds.
They are expected to add at least one more gold on the second and final day of the women's competition on Thursday when two-time gold medal winner Saori Yoshida defends her 55kg title.
After a flawless display, Icho, 28, said it was too soon to say if she would go for a fourth gold in Brazil in 2016.
"The last three Olympics have just run past so fast, probably the fourth will arrive fast too," she said. "But I have no idea at this moment."
Icho stayed remarkably composed despite becoming one of Japan's greatest Olympians. Asked repeatedly how it felt to win the hat-trick, she calmly replied: "I am very happy."
She never looked like losing her title. Her opponent injured her eye in an earlier bout and scraped through her semi-final.
Japan's Obara, 31, needed an ounce of luck to win gold at what she said would be her first and last Olympic Games.
Stadnyk of Azerbaijan took the first round and looked capable of beating the eight-time world champion. However, the Japanese wrestler fought back to win the final two rounds and take gold.
Obara, wearing a bright red leotard emblazoned with a roaring tiger's head, broke down after her victory.
"To be honest, I thought I would lose," she said.
"My coach said 'it's alright, it's alright' after the first period, so I was able to carry on."