Mayor Bill de Blasio carved out an hour from his official schedule to meet with his counterpart from a tiny city in South Carolina shortly after launching his quixotic presidential campaign.

De Blasio scored the endorsement of Orangeburg Mayor Michael Butler four days after their May 28 sit-down at Gracie Mansion — which lasted just as long as Hizzoner’s lone meeting that month with Council Speaker Corey Johnson.

Butler, whose city has a population of 12,903, is the country’s only municipal chief executive backing de Blasio’s White House bid.

De Blasio’s chat with Butler wasn’t announced and became public only after City Hall quietly posted Hizzoner’s official schedule for the day online.

A campaign finance expert said the revelation “raises serious questions about whether government property — Gracie Mansion — was used for campaign purposes.”

“It seems unlikely this conversation would be about anything other than his federal campaign,” said Alex Camarda of the Reinvent Albany government watchdog group.

De Blasio campaign spokeswoman Olivia Lapeyrolerie said the city Conflicts of Interest Board has told de Blasio that he can “use Gracie Mansion as he would use a personal residence.”

“Therefore, certain activities are allowed . . . that would not be normally allowed in city buildings,” she said.