Castex tested positive for Covid-19 on Monday evening, his office told AFP, adding that he would isolate for 10 days while continuing to work.
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 Gérald Darmanin and Europe Minister Clément Beaune.
The 56-year-old head of government received two doses of vaccine in the spring and had never tested positive before.
He has been criticised by some sections of the French press for not respecting mask-wearing guidelines – despite calling on the population to be careful.
De Croo and four of his ministers went into quarantine on Monday after the announcement of Castex’s positive test.
Since the start of the pandemic, 7.5 million people in France have contracted Covid and 119,000 have died as a result.