Chalmers won in 47.15 seconds and Jack Alexy of the United States surprised a stacked field to take silver, 0.16 seconds behind the winner. Frenchman Maxime Grousset grabbed bronze ahead of China's Pan Zhanle.
The heavily tattooed Chalmers appeared well off the pace at the start and was seventh out of eight swimmers at the turn, but he recovered to reel off his rivals and secure the victory in superb style.
"I feel awesome. That was the last medal that I had to have to complete the set in my career," said Chalmers, who has bagged the Olympic, Commonwealth and short course world titles in the event during his glittering career.
Romania's David Popovici, who boasts the world record of 46.86 seconds, was sixth in another disappointing run after his fourth-place finish in the 200m freestyle this week at the Marine Messe Fukuoka Hall.
Matthew Richards' hopes of a second gold after stunning 2022 champion Popovici in that event were also dashed, as the Briton could only manage fifth.
Frenchman Marchand captured his third individual title at the championships after his world record showing in the 400m individual medley and 200m butterfly gold, as he dominated the field to defend his 200m medley crown.
"I feel amazing these four days. Very big for me," said Marchand, whose explosive performances will make him the one to watch at his home Olympics next year.
"Very happy with the time and with the win today."
American Ryan Lochte's world record of 1:54.00 set at the 2011 Worlds in Shanghai appeared to be in danger after a good start, but Marchand dropped his pace slightly to finish with 1:54.82 for the third-fastest time in history.
"I will try my best," Marchand said when asked about his quest to lower the world record.
Duncan Scott was more than a second away for silver and fellow Briton Tom Dean settled for bronze.
FIRST GOLD
McIntosh earlier cruised to the women's 200m butterfly title, capturing gold in a time of 2:04.06 ahead of Australian Elizabeth Dekkers to win her first gold at the meet.
Regan Smith of the United States took the bronze but the race was all about 16-year-old McIntosh, who was 1.40 seconds ahead of her nearest rival and shattered her own world junior record en route to retaining the title she won in Budapest.
"I'm really happy with my race, 200m butterfly is definitely one of my favourite races," McIntosh said.
"Going into tonight I just wanted to see how hard I can push and how much I could hold on in that second half.
"I was just trying to make sure that I'm having as much fun as possible and enjoying the experience and process, and trying to feed off all the energy from all the people in the stands and it makes it a really fun time."
Australia's triple Olympic gold medallist Kaylee McKeown then won the women's 50m backstroke crown after claiming the title over 100m earlier this week.
The 22-year-old came home in 27.08 seconds ahead of American Smith who went 27.11 while Briton Lauren Cox had to settle for bronze, finishing 0.12 seconds behind the champion.
The Australian women's team then obliterated their own world record in the 4x200m freestyle relay as the quartet of Mollie O'Callaghan, Shayna Jack, Brianna Throssell and Ariarne Titmus cruised to victory in 7:37.50.
The Dolphins' side set the previous mark of 7:39.29 at the 2022 Commonwealth Games in Birmingham.
The USA women's team, gold medallists at five of the last six worlds in the event, finished second ahead of China.
Australia lead the medal tally with nine gold after five days of action.