GB's Tom Dean overcame two bouts of COVID-19 this year to win gold in the men's 200m freestyle, alongside Brit Duncan Scott who took silver.
Scott had qualified fastest for the final but it was his teammate Dean who held his form to win the second gold in the pool for Team GB at these games.
The bronze went to Brazil's Fernando Scheffer.
Dean, who twice contracted coronavirus last year, was third heading into the final 50 metres while Scott was down in sixth at the halfway stage, but the pair finished strongly.
"I knew it was going to be a dogfight, I didn't know how people were going to swim it, just race the race and that's how it is," Dean said.
Dean added that the Olympics seemed a "million miles off" when he was isolating in his flat after contracting COVID-19 twice in 12 months.
"I contracted COVID-19 twice in the last 12 months, I had six or seven weeks out during an Olympic year which is almost unheard of," he said.
"The first time wasn't too bad. The second time I did the full isolation period, I wasn't able to train and it was a slow-build back into training."
Dean's mother Jacquie Hughes told Sky News her son knew he could win.
"I knew he was feeling confident, I knew he had put in the work. He knew it was going to be a tight race - Duncan Scot is an amazing swimmer - and he knew, as he said, it was going to be a dog fight.
"I asked him this morning did you think you could win? And he said yes he did.
"He feels like he is in a dream at the moment."
In winning the gold, Dean set a new British record with a time of 1:44:22. Scott finished 0.04 seconds behind his fellow Brit.
Scott was narrowly above Dean in the rankings coming into the Games and qualified fastest but was delighted for his teammate.
"Just a massive credit to Tom Dean. That was unbelievable. Olympic champion," he said.
Around 70 members of Dean's family celebrated the win at the family home in Maidenhead, Berkshire.
Peaty, who cheered on Dean and Scott, congratulated the pair on Twitter, he said: "Great to see such a force of talent on the team, absolutely sensational."
"To come along so far in the last 18 months, it's a pleasure to watch him.
"It's great to be able to say he's a good mate out of the pool."
This result marks the first time two British male swimmers have shared an Olympic podium since London 1908 when Henry Taylor and Tom Battersby won gold and silver in the 1500m freestyle.
There was heartbreak for Team GB's Bianca Walkden in the women's 67kg Taekwondo competition.
The 29-year-old from Liverpool lost her semi-final after taking a head kick from South Korea's Lee Da-Bin in the closing seconds.
She will compete for the bronze medal later today.
In the women's kayak single event, Britain's Kimberley Woods managed to secure a place in the final after qualifying in second place in her semi-final behind Slovakia's Eliska Mintalova.
But there was disappointment for Woods who finished outside medal contention in the final.
There was a surprising result in the tennis, which saw Japan's Naomi Osaka lose out on a gold medal to former French Open finalist Marketa Vondrousova of the Czech Republic 6-1, 6-4 in the third round of the women's singles.
Osaka, who lit the Olympic cauldron in Friday's Tokyo Games opening ceremony, won her opening two matches in straight sets.
Elsewhere in the 50m women's backstroke, Brit Kathleen Dawson finished in sixth place, with Kaylee McKeown of Australia taking gold and breaking the Olympic record.
In the women's 200m freestyle semi-final, Brit Freya Anderson missed out on a spot in the final after finishing seventh in her heat.
There was a shock in the women's 100m, with 17-year-old Lydia Jacoby from the United States taking gold - the first time someone from Alaska has appeared in the Olympics.