The 17-year-old midfielder discussed signing for the Blues last month, when he watched their Carabao Cup win over Newcastle from the directors' box.
It was mooted that he could sign for Chelsea then head on loan to their BlueCo sister club Strasbourg, however he will instead move directly to French team this summer.
Diong was spotted playing for AF Darou Salam in his home country and will add to a long list of young stars under the BlueCo umbrella.
Chelsea have secured Ecuadorian teenager Kendry Paez, Jamaica international Dujuan Richards and Strasbourg loanee Angelo Gabriel among the youthful talents they have pursued.
Strasbourg, meanwhile, are formulating plans to field an entirely under-23 line-up with significant sums forked out for Abakar Sylla, 21, and Emanuel Emegha, 20, during the summer.
Patrick Vieira's side sit ninth in the Ligue 1 table and are on a seven-game unbeaten run in all competitions.