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.
At least 14 killed in explosion at weapons depot in Syria
At least 14 people were killed and 11 wounded on Wednesday in an explosion at a temporary site used to store weapons and remnants of war near the Syrian city of Sarmada in Idlib province, state television reported.
US hits five Iranian oil tankers after attack on Navy ships
The United States has destroyed five Iranian oil carriers on Tuesday after the Islamic Revolutionary Guard Corps (IRGC) targeted its navy warship with ballistic missiles "twice over the past two days".
UK gives air traffic provider a week to probe outage that halted flights
Britain's NATS air traffic control provider has a week to investigate the systems failure that halted flights and stranded hundreds of thousands of passengers, the government said, as pressure mounts on its chief executive.
Serbia's President Vucic dissolves parliament, calls snap elections
Serbia's President Aleksandar Vucic on Wednesday dissolved parliament and announced snap elections for next month as he seeks to extend his hold on power by running for prime minister.
With large crowds attending, Norway lays King Harald to rest
Norway's King Harald Vwas being laid to rest on Wednesday, in a series of ceremonies marking the culmination of 13 days of national mourning following his death at the age of 89.
Jordan intercepts 18 Iranian ballistic missiles
Jordanian army forces have intercepted 18 Iranian missiles targeting the country early on Wednesday.
Air traffic control issue disrupts flights at major UK airports
Flights at major British airports were disrupted on Tuesday by a technical fault at air traffic control provider NATS, the latest in a series of failures that renewed questions about the resilience of Britain's aviation infrastructure.
At least 73 injured after Houthi rebels attack Saudi civilian and energy facilities
At least 73 civilians have been injured after Yemen's Houthi rebels attacked a number of civilian and energy facilities in Saudi Arabia's southern region on Tuesday, state media confirmed.
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Pinoy Expat Corner
Why did they choose mountain biking as their sport? What makes mountain biking different from regular road cycling?
Pinoy Expat Corner
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Pinoy Expat Corner
Why is it important for them to preserve Ilokano culture and traditions even while living far from the Philippines?
