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.
Saudi Arabia intercepts and destroys drone in Mecca region
Saudi Arabia's air defences intercepted and destroyed a drone from the Houthis targeting the Mecca region on Friday morning.
Six feared dead in helicopter crash in Indian Himalayan state, media report
Six people were feared dead in a helicopter crash in the Indian Himalayan state of Uttarakhand on Friday, television channels reported, adding that bad weather was suspected to be the cause.
Russia, Ukraine say aid is entering Oleshky, residents can leave if they wish
Desperate residents of a Russian-held town in southern Ukraine were due to receive food and medical supplies on Friday and be allowed to leave if they wish, officials from the two warring sides said.
Four killed as Italian helicopter in EU mission in Bosnia crashes
Four people died and 11 were injured after an Italian military helicopter crashed in Bosnia and Herzegovina on Thursday, Italy's Defence ministry said.
The UAE has confirmed that the oil spill from the tanker GEM No. 2 has been contained, as authorities continue environmental remediation efforts along affected coastlines.
His Highness Sheikh Mohammed bin Rashid Al Maktoum, Vice President and Prime Minister of the UAE and Ruler of Dubai, has reviewed plans and implementation stages of the AED 500 million Arab Reading Challenge Endowment project, which will be developed by Sobha Realty in Dubai.
The UAE's Military Attorney-General has ordered the referral of a number of defendants to the Military Criminal Court over allegations of abusing official authority to unlawfully obtain exemptions from national and reserve service.
