Ilias Iliadis won the host nation's second gold of the Athens Games with victory in the men's 81kg category.
Iliadis, the European champion, beat Ukraine's Roman Gontyuk in the final to land Greece's first ever judo title.
"I can't believe what's happened and I want to thank my coach as it's all down to him," said the Georgian-born Ilias, whose coach is also his father.
"It's the best feeling a father could ever have - I felt tense until the last moment," said Nikos Iliadis.
Iliadis made a dream start to the final, scoring waz-ari with a foot sweep after just 10 seconds and adding yuko 25 seconds later with a pick-up.
Iliadis then scored ippon with soei-otoshi after just over two minutes.
Bloodied and injured Russian Dmitri Nossov and Brazil's Flavio Canto took the bronze medals.
Nossov beat Mehman Azizov of Azerbaijan for his bronze, while Canto beat Poland's Robert Krawczyk.
Meanwhile, Ayumi Tanimoto beat Austria's Claudia Heill to win the women's 63kg category and claim Japan's fourth gold medal of the judo tournament.
Tanimoto was a convincing winner, throwing Heill for waza-ari with seoi-nage and then pinning her to claim the gold.
Urska Zolnir claimed bronze against Marie Chisholm of Canada and Cuba's Driulys Gonzalez won the other bronze by walkover after Argentine world champion Daniela Krukower withdrew through injury.