Tunisia's Ahmed Hafnaoui was so shocked to see his name in the gold medal position for the 400m freestyle on Sunday he did not believe his eyes but after letting his stunning win at the Tokyo Olympics sink in he said he hoped to have made his family proud.

The 18-year-old, who finished ahead of Australia's Jack McLoughlin and U.S. swimmer Kieran Smith, said he was surprised even to be in the final, where he swam in the outside lane after being the slowest in qualifying.

"I believe it when I touched the wall ... I was so surprised I didn't accept that," he told a news conference.

"I dedicate (the gold) to all my family, my mum, my dad, my sisters, I wish they are proud of me."

His gold medal is only the fifth won by a Tunisian athlete at the Olympics, but their third in swimming.

Hafnaoui powered home over the last 50m, touching the wall in 3:43.36, far quicker than the 3:45.68 he swam in qualifying on Saturday. Letting out a scream when he realised he had won, he pumped his first and pointed at the electronic score board.

"I just can't believe it. It's a dream and it became true. It was great. it was my best race ever," he said.

Hafnaoui, who joined Tunisia's national swimming programme as a 12-year-old, was all smiles at the medal ceremony and news conference where he was mobbed by reporters and inundated by requests for selfies.

Hafnaoui heaped praise on his coaches, saying they had been so supportive and had "worked too hard".

Hafnaoui, whose father Mohamed Hafnaoui was a former member of the national basketball team, will race in the 800m freestyle on Tuesday. He said he plans to go to college in the United States but did not specify where.

"I felt better in the water this morning than yesterday and that's it. I'm the Olympic champion now."

When asked what he does at home, the teenager said "normal things", like hanging out with friends and playing football.

Smith, who claimed bronze, said he knew nothing about the Tunisian coming into the race.

"He's the Olympic champion in the 400 metre freestyle. That's all I know about him," said the American. "I'm very proud of him."