STORY: Downing Street shared a photo of Sunak, a Hindu and Britain's first government leader of Indian origin, lighting a traditional oil lamp at the event.
Diyas are oil lamps traditionally lit during Diwali.
The prime minister's residence regularly hosts a reception to mark the annual Hindu festival of lights.
"Brilliant to drop into tonight’s Diwali reception in No10," Sunak wrote on Twitter, "I will do everything I can in this job to build a Britain where our children and our grandchildren can light their Diyas and look to the future with hope."
Sunak came to office on Tuesday (October 25) facing a daunting list of problems, including an economic crisis and a divided Conservative party shredded by months of scandals and rows.