The award adds to a successful run at the French cinema showcase for Asian films after Japanese director Hirokazu Kore-eda clinched the prestigious gong last year.
Bong, who made his mark at Cannes in 2017 with Netflix-produced Okja, set his latest movie in modern South Korea.
It follows a down-on-their-luck family of four who spot an opportunity to con a wealthy household into giving them jobs.
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They worm their way into the other family’s lives – before things start going south.
This year’s Cannes film festival also shone a light on newcomers, in an unusually crowded field.
Atlantics, a ghost story about migrants directed by Franco-Senegalese filmmaker Mati Diop, won the runner-up Grand Prix award.
The movie, based on her 2009 short documentary, was Diop’s first feature-length film.
Spain’s Antonio Banderas won the male acting prize for his role as a tortured filmmaker in Pedro Almodovar’s loosely biographical Pain And Glory – one of the films that had been tipped for the top honour.
Britain’s Emily Beecham was crowned best actress after starring in Jessica Hausner’s Little Joe as a botanist who starts having doubts about her latest genetically-modified creation when it begins to affect her loved ones.