The two-time world champion defeated Russia's Denis Albyazin to take the top spot, while Japan's Kenzo Shirai took bronze. The two other winners' broad smiles contrasted dramatically with Ri's look of disappointment.
Some viewers claimed on social media Ri Se-Gwang appeared so sad because he now faces the prospect of returning to Pyongyang and the control of leader Kim Jong-un.
See more photos after the cut.
Officials from North Korea had accompanied the North Korean athletes to Brazil, on the strict orders of the country's government. They were this week reported to have confiscated Samsung mobiles phones handed out free to competitors due to concerns they would try to contact the outside world.
The electronics giant is one of the official sponsors of the Rio Games and handed out free Galaxy S7 handsets to all 11,200 participants to use at the opening ceremony in the Maracana Stadium.
However, it has emerged that the North Korean team were not allowed to pick up a free phone after the devices were banned by the North Korean Olympic committee.
And competitors have also been banned from visiting tourist attractions or mixing with other athletes at the Games.