Four other people have also been placed under formal investigation for extortion and criminal association, judicial sources told news agencies Reuters and Agence France-Presse.
Mathias Pogba's lawyer, Yassine Bouzrou, said his client is innocent.
He told French broadcaster BFMTV: "We are going to challenge this decision."
Mathias Pogba, 32, has admitted he was behind a video that appeared online last month promising "revelations" about the 29-year-old Juventus midfielder.
French prosecutors opened a judicial investigation earlier this month after former Manchester United player Paul said he was the target of extortion and threats from an organised gang.
He filed a complaint with Turin prosecutors on 16 July claiming he was the target of a 13m euro (£11.29m) blackmail plot.
A World Cup winner with France in 2018, Pogba moved to Juventus on a free transfer this summer after leaving Manchester United.
Mathias Pogba is also a professional footballer who has been capped by Guinea and played for clubs across Europe including Crewe, Wrexham, Crawley and Partick Thistle.