US teen swimmer Lydia Jacoby has won a shock gold medal in the women’s 100-metre breaststroke final at the Tokyo Olympics, leading to her family members and friends breaking into celebration at her hometown in Seward, Alaska.

       The 17-year-old champion has become the first American woman to secure a gold in swimming at the Tokyo Olympics with her stellar debut performance.

       Jacoby beat South Africa’s Tatjana Schoenmaker and US teammate Lilly King, who grabbed silver and gold respectively at the competition.

       The new swimming star finished the winning lap in 1:04.95, knocking off Schoenmaker by just .27 seconds.

       King, an Olympics Champion with three medals in her career at the Games, took the bronze for the US after trailing Jacoby by .59 of a second.

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       Jacoby appeared to be in shock when she turned back to look at the results after completing the final 50 metres and touching the wall.

       “I was definitely racing for a medal," Jacoby said after finishing her race. "I wasn’t really expecting a gold medal so when I looked up at the scoreboard, it was insane.”

       Her schoolmates were seen having an energetic celebration at a watch party in her hometown after her debut performance.

       “It’s pure joy. The whole town. One of the local restaurants, Zudy’s, they’re selling the ‘Go Lydia, go!’ stickers. And it’s a fundraiser for the club,” Sarah Spanos, whose sons trained with Jacoby, told nonprofit media organisation Alaska Public Media.

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       Jacoby is also the first Alaskan swimmer ever to win a swimming gold medal at the Olympics.

       Her parents are both boat captains. She took up swimming because they wanted her to be safe in the water with them.