Belgium s Prime Minister Alexander De Croo (R) and his French counterpart Jean Castex (L) address a press conference after a bilateral ministerial meeting on November 22, 2021 in Brussels. AFP
Castex "immediately carried out a PCR test" after finding out that one of his daughters had tested positive, his office said.
Castex had been in Brussels on Monday morning, where he met his Belgian counterpart Alexander De Croo.
He was accompanied on the trip by several other senior colleagues, including France's Interior Minister Gerard Darmanin and Europe Minister Clement Beaune.
The 56-year-old head of government received two doses of vaccine in the spring and had never tested positive before.
President Emmanuel Macron and his wife Brigitte both contracted the disease last year.
Meanwhile, Belgian Prime Minister Alexander De Croo and four of his ministers went into quarantine on Monday after French Prime Minister Jean Castex tested positive for Covid-19, a government statement said.
"They will also take PCR tests and will remain in quarantine until the test results come back negative," the statement said, hours after they attended a meeting with Castex and other senior French officials.