The Seoul Metropolitan Police Agency said on Thursday that officers were searching for him around Sungbuk-dong, a district in northern Seoul, where his phone signal was last detected.
His daughter reported him missing at 5:17 p.m. (0817 GMT) and said his phone was off.
Park left the official residence at around 10:40 a.m., wearing a black hat and a backpack, having canceled a policy meeting that was scheduled for Thursday morning, according to multiple local reports.
Kim Ji-hyeong, an official from the Seoul Metropolitan Government, confirmed that Park had not shown up for work Thursday because of unspecified reasons and had canceled all his scheduled activities, including a meeting with a presidential official at his Seoul City Hall office.
Park has been the mayor of Seoul since 2011.
A member of President Moon Jae-in’s liberal Democratic Party, Park is seen as a potential presidential hopeful in the 2022 presidential elections.