Diemers, 26, had an agreement in principle to join Groningen at the end of the season.
However, Feyenoord signaled their interest and that changed everything, giving Diemers a big decision to make.
The midfielder’s agreement with Groningen expires today and he will instead be moving to De Kuip, AD reports.
A key performer for Fortuna Sittard this season, Diemers scored seven goals and provided six assists.
Diemers’ arrival is a big boost for coach Dick Advocaat, who has seen several midfielders leave since he took over.
Yassin Ayoub, Liam Kelly, Renato Tapia and Oguzhan Ozyakup have all gone, while Leroy Fer has not renewed his expiring contract.
Advocaat wants at least six midfielders because Feyenoord will be in next season’s Europa League group stage.