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.
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US judge permanently blocks release of report on Trump documents case
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Former UK ambassador Mandelson released on bail after Epstein revelations
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Duterte was 'pivotal' in murder of thousands, ICC prosecutors say
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