The 11th seed claimed the Olympic Games crown with a golden score win over Cuban Asley Gonzalez.
Both fighters picked up warnings for passivity, before the Republic of Korea coach was sent out for shouting instructions during play as the tussle headed into the final 30 seconds.
Song then won the gold with a reverse leg sweep to claim a Wazari on golden score – and his coach ran back into the arena at the ExCeL in wild celebration.
World champion and top seed Ilias Iliadis – who won the -81kg title at the Athens 2004 Olympics – was beaten in the quarter-finals, but fought back to take bronze by edging past Tiago Camilo of Brazil, who had twice made the podium at previous Games.
Masashi Nishiyama of Japan, the second seed, claimed the other bronze by beating defeated semi-finalist Kirill Denisov of Russia on the judges' flags.