The 16-year-old France Under-17 international midfielder has joined from Metz.
Speaking earlier this month, Kemen said: "Newcastle is a big team, and they look after young players. If they agree everything with Metz, I will join them.
"There are some good French speaking people there, so I can settle there fine on the guidance of everyone there."
Newcastle have also confirmed that Paul Dummett, Conor Newton - both of whom were on loan at St Mirren last season - Michael Richardson, Alex Gilliead plus goalkeepers Jak Alnwick and Jonathan Mitchell have all signed new deals.