The 23-year old midfielder only joined the Black Cats from Bursaspor in January but manager Paolo Di Canio has sent him back to Turkey after 16 appearances in England last term.
Di Canio has brought in Emanuele Giaccherini, Cabral and El-Hadji Ba this summer meaning N'Diaye has fallen right down the pecking order.
A club statement read: "Alfred N'Diaye has joined Eskisehirspor on a season-long loan.
"The 23-year old returns to Turkey having arrived at the Stadium of Light from Bursaspor during the January transfer window.
"N'Diaye, a France youth international, has made 16 appearances for Sunderland."