The French defender Giulian Biancone has joined the Forest.
The fee is said to be a record for the team.
Biancone joined Troyes in the summer of 2016 after spending two years on loan with Belgian club Cercle Brugge.
Steve Cooper said that he is a very strong addition at centre-back or on the right.
Cooper led the City Ground side to promotion into the top tier of English football.
Biancone is Forest's third major signing of the summer after Dean Henderson and Taiwo Awonyi.
He played 33 times for Troyes and scored once.
"After speaking to the head coach, it's clear that there are exciting times ahead and I'm looking forward to joining up with my team-mates and getting started in the premier league," said Biancone.