Ri Se-Gwang with silver medallist Denis Abliazin of Russia and bronze medallist Kenzo Shirai of Japan. Spot the odd one out.

Winning an Olympic gold medal should have been one of the happiest moments of Ri Se-Gwang's life.

But the North Korean gymnast looked like he'd rather be doing almost anything else, as he stood stony-faced atop the podium in Rio de Janeiro.

Ri's frown was a stark contrast to the smiles of Russian silver medallist Denis Albyazin and Japanese bronze medallist Kenzo Shirai.

MIKE BLAKE/REUTERS North Korea's Ri Se-Gwang says he's "filled with joy" at winning a gold medal for gymnastics, but he really doesn't show it.

He's been dubbed "the saddest gold medallist", and sparked speculation over why he looked so glum, despite becoming the third North Korean gymnast in history to win Olympic gold.

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MARKO DJURICA/REUTERS Ri Se-Gwang holds the North Korean flag after his "joyous" win.

Some viewers suggested Ri might be miserable about the thought of having to return to the dystopia of North Korea, and the dictatorship of Kim Jong-un.

But Ri - who gave a military salute after being awarded his medal - insisted he was thrilled to win.

"I was filled with joy because I thought I was able to bring a sense of victory to our leader," he was quoted by the Daily Mail as saying.

EDGARD GARRIDO/REUTERS North Korea's Kim Song Guk (right) sort of smiled after winning bronze, and was praised for congratulating South Korean gold medallist Jin Jong Oh.

The newspaper reported that North Korean government officials, sent to Rio to accompany the Olympic team, had confiscated their free cellphones, given out by Samsung, to prevent the athletes contacting anyone.

The athletes were also reportedly banned from visiting tourist sites or spending time with athletes from other teams.

Nonetheless, two of Ri's teammates have been praised for their positive interactions with South Korean competitors.

DYLAN MARTINEZ/REUTERS North Korean gymnast Hong Un Jong put on a happy face for a selfie with South Korea's Lee Eun-Ju.

Shooting bronze medallist Kim Song Guk congratulated South Korean gold medallist Jin Jong Oh on his win, and spoke of reunifying the two Koreas, telling media: "If the two [Koreas] become one, we could have a bigger medal. And if both the gold and bronze medals came from one Korea, it could have been a much bigger win."

North Korean gymnast Hong Un-Jong posed for a happy selfie with South Korean competitor Lee Eun-ju earlier in the Games.

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