It's after the resignation of Edouard Philippe who has been in the post for three years.
Mr Castex, 55, is relatively unknown in France, although he is a senior civil servant.
The new Prime Minister has been a key player in the government's response to the Covid-19 pandemic.
Israeli strikes kill two in Gaza, destroy medicine storage warehouses
Israel's military on Saturday killed two Palestinians in Gaza and destroyed two warehouses near a hospital that stored medicine in separate strikes, amid uncertainty over whether Israel has accepted a deal reached with Hamas over the group's disarmament.
Restaurant explosion in central Moscow kills three, injures 15
Three people were killed and 15 injured in an explosion that went off near a summer restaurant terrace in central Moscow on Saturday evening, the Russian capital's police force said.
Thirteen die in tourist plane crash over Peru's Nazca Lines
A small tourist plane crashed on Saturday as it flew over the Nazca Lines archaeological site in southern Peru leaving 13 dead, Peruvian state news channel TVPeru said.
All 10 climbers confirmed killed in Pakistan avalanche
Celebrated Nepali mountaineer Nirmal Purja and nine others were confirmed dead on Saturday after an avalanche on Broad Peak in northern Pakistan.
UAE President His Highness Sheikh Mohamed bin Zayed Al Nahyan held talks with Ukrainian President Volodymyr Zelenskyy on Sunday, focusing on bilateral cooperation and the latest developments in the Ukraine conflict.
The United Arab Emirates has condemned the Iranian hostile attack targeting Kuwait with drones.
The United Arab Emirates has expressed its condolences to South Africa following a deadly bus crash in the Free State province that killed at least 12 people, including five children, and left dozens more injured.
