President Rodrigo Duterte said he asked ambassador Teodoro Locsin to replace Alan Peter Cayetano, who planned to seek a seat in the lower house of Congress. Cayetano was a three-term congressman and a two-term senator before he joined the cabinet last year.
"Can you please call Teddy Boy Locsin and ask him if I can mention his name here," Duterte said to an aide during a news conference in Manila.
Locsin, a former three-term congressman and a newspaper publisher, tweeted later on Friday: "I was asked. I said yes."
Candidates began registering on Thursday for the mid-term elections scheduled for May 2019 and seen as a referendum on Duterte's administration.
Five more cabinet members are expected to quit and run for elective positions, including the ministers for agriculture and agrarian reform, a political adviser and a special assistant.
Nearly 320 seats in the two-chamber legislature and more than 17,000 local government positions, from 81 provincial governors to about 1,600 municipal councillors, will be contested nationwide.
(With inputs from agencies.)