The defeat ended U.S. hopes of their first men's champion since 2004 and left 19-year-old Stevenson distraught at the loss.
The American left the ring and avoided the media to crouch, sobbing, with a towel over his head at the most stinging loss of his budding career.
"He's not distraught by the decision, just distraught by his loss," U.S. coach Billy Walsh told Reuters. "This kid has very seldom lost in his life, he's been youth Olympic champion and he's won everything.
"It's hard for him to take, to be on the other end of a defeat, especially in the Olympic final."
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The gold was Cuba's second of the boxing tournament after Julio Cesar La Cruz secured the light-heavyweight title on Thursday.
Russia's Vladimir Nikitin and Uzbekistan's Murodjon Akhmadaliev took the bronze medals.
Robeisy, whose path to the Games was far more circuitous than Stevenson's with the 22-year-old Cuban having to travel to Azerbaijan in June to qualify after failing to secure a slot in the Americas, showed his pedigree and craft.
He won the first round but lost the second to go into the third with the scores level.
The three scorecards that counted had him winning the third 10-9 10-9 9-10.
Walsh said Stevenson had fallen short on work rate.
"He had good tactics, he was working well and just needed to throw more punches and when his opponent attacked he needed to meet him," he said. "He didn't do that, he backed off.
"It's the second time he (Ramirez) has won at a different weight division so he's an exceptionally talented boxer and it was a big ask. It was a man against a boy," said Walsh.
The U.S. team have one more shot at gold in the competition with Claressa Shields fighting to defend her women's middleweight crown on Sunday.
(Editing by Clare Fallon)
