Hopkin was part of the quartet who won gold in the inaugural mixed 4 x 100 metres medley relay final in a new world record time.
The original quartet of Kathleen Dawson, Adam Peaty, James Guy and Freya Anderson were more than two seconds clear of the rest of the field in the heats, setting a new European record.
And Team GB, with the addition of Hopkin for Anderson, eased to victory on Saturday morning.
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Hopkin replaced Anderson for the final freestyle leg and touched out in three minutes and 37.58 seconds for Britain’s fourth gold in the pool at Tokyo 2020 and seventh gong overall – matching their tally from London 1908.
The time was 0.83 seconds better than the previous best benchmark of 3mins and 38.41ecs – set by China last year – while they finished 1.28secs clear of China, with Australia taking bronze.
Hopkin said: “We were saying, before the medal ceremony, maybe once we get the medal it will feel real.
“But I don’t think it’s quite sunk in yet.”
This event has been added to the Olympics schedule for the first time – where two males and two females must be selected but the nation can use any combination in the backstroke, breaststroke, butterfly and freestyle splits.
It means the lead can change hands multiple times as men and women can race against each other in the same leg of an event that was first introduced for long course swimming at the 2015 World Championship.
Dawson started with the backstroke and her time of 58.8s meant Peaty leapt into the pool with Britain sixth, and the 100m breaststroke gold medallist’s astonishing split of 56.78s helped them up into fourth.
Guy, who won his first gold earlier this week in the men’s 4 x 200m freestyle relay, clocked exactly 50s in the butterfly to take Britain into the lead and Hopkin finished in 52s flat to spark exuberant celebrations.