The Iranian, who only managed 16th place in Beijing four years ago, beat Russia's Rustam Totrov in the final of the 96kg weight class.
Jubilant Iranian supporters roared and waved their nation's red, white and green flags as Rezaei kissed and hugged his trainer in celebration.
Rezaei had beaten the Armenian favourite Artur Aleksanyan in the quarter-final and Cuba's Yunior Estrada Falcon in the semi-final.
Totrov won bronze at the world championships in 2011 and was among the favourites for gold in London.
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