The gold medal was Shirai's fifth in world championship competition and third medal in Montreal. He won his third consecutive floor exercise title on Saturday after claiming the all-round bronze medal on Thursday.
Korean Rio Olympic vault champion Yang Hak-seon withdrew from the final.
Mai Murakami added a second gold for Japan by winning the women's floor exercise with a score of 14.233.
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The podium finish was her first in Montreal after two fourth places.
American Jade Carey (14.200) earned the silver and the bronze went to Britain's Claudia Fragapane (13.933).
China continued their strong showing at the championships when Zou Jingyuan defeated Olympic champion Oleg Verniaiev of Ukraine on the parallel bars.
Zou won his first world medal with a score of 15.900 to Verniaiev's 15.833. Russia's David Belyavskiy (15.266) earned the bronze.
Germany took gold and bronze in the women's beam with Pauline Schaefer the winner with a score of 13.533. She had won the bronze in 2015.
This year's bronze went to countrywoman Tabea Alt (13.300) with all-round champion Morgan Hurd of the United States the silver medallist (13.400).
Croatia's Tin Srbic defeated 2012 Olympic champion Epke Zonderland and fellow Dutchman Bart Deurloo to earn the horizontal bar gold medal. Srbic's score of 14.433 topped Zonderland's 14.233. The bronze was won by Deurloo in 14.200.
(Reporting by Gene Cherry in Salvo, North Carolina; Editing by Rex Gowar)
