As part of the deal, Milambo will join the Bees for £17 million, with the potential for an additional £4.25 million in add-ons.
During his time at Feyenoord, Milambo made 42 appearances in all competitions, scoring seven goals and providing eight assists last season.
Head coach Keith Andrews expressed his excitement about the new signing, stating that Milambo has the potential to be a great Premier League player.
Andrews praised Milambo's skills, saying that he is a forward-thinking midfielder who can score different types of goals.
Technical director Lee Dykes also welcomed Milambo's arrival, highlighting his ability to develop his game to a high level in England.
Dykes expressed his appreciation to Feyenoord's senior management team for their assistance and professionalism throughout the transfer process.
With Milambo on board, the Bees hope to strengthen their midfield and make a push for European football in the new campaign.