Crismanich, who won the Pan American qualifiers for London, beat Hemme 1-0 in a cagey final to win Argentina's first Olympic taekwondo medal. He scored the deciding point with seconds remaining then survived an onslaught to seal the gold.
"I can't express with words what I feel and what is happening to me," he said. "It's a dream. I am really happy."
Britain's Lutalo Muhammad, controversially selected in place of world number one Aaron Cook, lost to Hemme in the quarter-finals but made it through the repechage competition to the bronze medal match, where he faced Armenia's Armen Yeremyan.
The Londoner's power and range of kicks earned him a 9-3 win and gave Britain's men their first Olympic medal in taekwondo. Cook missed out on a bronze medal in Beijing when he lost the repechage to China's Zhu Guo.
Muhammad said he did not use the furore over his selection to fire himself up.
"It was a tough ordeal coming in. But I don't use it as fuel or anything like that," he told reporters. "I'm just very proud to win a medal for Team GB. It was always going to be great to win a medal.
"Being in London makes it sweet. But I really wanted to deliver and bring that gold medal home. That's unfortunate, I was unable to do that. But I'm happy to get the bronze."
Sebastián is the first individual athlete achieving the gold medal for Argentina since 1948, and the first taekwondo winning a medal for Argentina. His brother, Mauro, is also a well-known taekwondo practitioner who won a bronze medal in the men's flyweight (under 58 kg) at the 2009 World Taekwondo Championships in Copenhagen.