The 20-year-old also set a new British record as he finished just behind China’s Olympic champion Sun Yang with a time of three minutes 43.75 seconds.
Guy, who qualified second-fastest for the final, had gone off in front from the start and only Sun could keep up, before pulling ahead and taking the gold.
“Everyone wants to win obviously,” said Guy. “But I can’t be happier to win a medal at the World Championships – I’m over the moon with that one.
“I tried to race that from the start and back-end it. Sun Yang is a distance swimmer so he has more endurance than I do so my best chance was to go out hard and try and hold on.
“It was stroke for stroke and that’s what it will be like next year as well, maybe with a couple more guys in there – so that will be the big one.”
Great Britain will be hopeful of more success today as five made it into their respective finals. World record holder Adam Peaty eased into the 100m breaststroke final, as did Scotland’s Ross Murdoch, whilst Ben Proud was second in his semi-final to make it into the 50m butterfly medal race. Siobhan-Marie O’Connor was untouchable in the 200m individual medley semi-final and Inverurie’s Hannah Miley also qualified.
Katie Ledecky, of the United States, opened an ambitious programme with an easy victory in the 400m freestyle.
The 18-year-old Ledecky led all the way and was under her world-record pace until the next-to-last lap, touching in 3 minutes, 59.13 seconds to set a championship mark. Ledecky set the world record of 3:58.37 at last year’s Pan Pacific championships in Australia.
Ledecky will be favoured in the 800 and 1,500 freestyles and is a threat in the 200 free. Sharon van Rouwendaal, of the Netherlands, took silver in 4:03.02. Jessica Ashwood, of Australia, earned bronze in 4:03.34.
France outfought Russia to win the men’s 4x100 free relay in 3:10.74. Mehdy Metella, Florent Manaudou, Fabien Gilot and Jeremy Stravius took the lead on the third lap and hung on by 0.45 for gold.
Cheered by the near-sellout home crowd, Russia’s team of Andrey Grechin, Nikita Lobintsev, Vladimir Morozov and Alexander Sukhorukov touched second in 3:11.19. Luca Dotto, Marco Orsi, Michele Santucci and Filippo Magnini, of Italy, earned bronze in 3:12.53.
Australia won the women’s 4x100 free relay in a championship-record time of 3:31.48. Emily Seebohm, Emma McKeon and sisters Bronte and Cate Campbell took the lead for good at the 200m mark. Ranomi Kromowidjojo, Maud van deer Meer, Marrit Steenbergen and Femke Heemskerk, of the Netherlands, finished second in 3:33.67.
Sarah Sjoestrom, of Sweden, set a world record in the semi-finals of the 100m butterfly. Competing in the second semi, she won in 55.74 seconds to lower the old mark of 55.98 set by American Dana Vollmer at the 2012 London Olympics. The final is tonight.