Peaty cruised to his previous two Olympic golds, in 2016 and 2021, but has struggled with injury and his mental health since the last Games in Tokyo.
He took a break from the sport and considered stopping entirely and in his absence at last year’s World Championships Qin emerged as a serious rival.
Qin swam a 57.69-second 100m last year, making him the second-fastest of all time behind Peaty and the quickest of anyone since Tokyo – but the Uttoxeter swimmer’s 57.94 at the British trials signalled a return to form.
Although he was well adrift of that time at an electric La Defense Arena in Paris, he was the more comfortable of the two in the semi-finals and looked content as he nodded towards the big screen after beating his rivals.
“If you’re Peaty watching this you’re thinking ‘this is mine tomorrow’,” said Adrian Moorhouse, Britain’s 1988 gold medallist in the event.
“You have got to be. Qin is not on it. The rest of the field caught him. He didn’t have enough.”
Britain’s James Wilby was fifth in Peaty’s race and missed out on the final.
In the relays, the male British quartet qualified for the final third-fastest but were never really in contention as the United States cruised to gold, Australia took silver and Italy bronze.
Matt Richards, Tom Dean and Duncan Scott, who made up the quartet along with Jacob Whittle, will be favourites to defend their 4x200m freestyle title on Tuesday.
Australia outclassed the US to win gold in the women’s 4x100m freestyle.
That was Australia’s second gold of the night after Titmus earlier denied American Katie Ledecky an eighth Olympic gold, which would have equalled the record for a female swimmer.
Canada’s 17-year-old sensation Summer McIntosh took silver and Ledecky bronze.