The full-back has reached an agreement with the Rotterdam giants, and will return to his boyhood club in the summer on the Bosman ruling
The Rotterdam side were in the market for a new right-back due to the expected departure of Dani Fernandez and Gill Swerts at the end of the season, and they see the 22-year-old Janmaat as the perfect addition.
Janmaat is a product of the Feyenoord youth academy, but never made it into the first-team in Rotterdam. He enjoyed a successful spell with ADO Den Haag before joining Heerenveen in the summer of 2008.
He had since developed into a key player at the Friezen, thus earning himself a move back to his boyhood club.
Janmaat is Feyenoord's third signing for the summer transfer window after previously securing the services of NEC midfielder John Goossens and Roda star Ruud Vormer on Bosman moves.