The swearing-in ceremony was held by Taiwanese President Tsai Ing-wen, and it took place in the presidential administration building. Members of the new cabinet, including ministers, counsellors and department heads, were also sworn in.
On Friday, President Tsai appointed Su, the former head of the ruling Democratic Progressive Party (DPP), as the country's new prime minister.
Lai initially announced his decision to step down soon after the defeat. However, Tsai — who herself resigned as the chair of the DPP — refused to accept his resignation back then.
Su already served as the Taiwanese prime minister between 2006 and 2007 before serving as the chairman of the DPP from 2012 to 2014.