The 24-year-old Swiss winger is expected to leave Italian club Bologna, where he has been playing, to join Forest.
According to reports, Ndoye has signed a five-year deal with Forest, with an option to extend it by another 12 months.
Nottingham Forest will pay €40 million for Ndoye's transfer, plus an additional €5 million in add-ons, and will also give Bologna a 10% share of any future sell-on fee.
Ndoye's move to Forest comes after Napoli previously expressed interest in signing him, but failed to reach an agreement with Bologna.
