Guessand says he jumped at chance to join Aston Villa after sealing move
The Ivory Coast forward will reportedly cost Villa an initial £26 million ($35 million) with a further £4.3 million in add-ons.
Guessand scored 13 goals and registered 10 assists in all competitions as Nice finished fourth in Ligue 1 last season.
The 24-year-old bolsters Villa's attacking options after the sale of Jhon Duran in January to Al-Nassr and the departures of loanees Marcus Rashford and Marco Asensio, who played a starring role in the second half of last season.
"When I heard the club wanted me I didn't think twice," Guessand said. "I'm ready to give my all. Every game, every kick, every minute."
Villa have had a quiet transfer window until now, signing only back-up goalkeeper Marco Bizot and defender Yasin Ozcan, who has been loaned to Anderlecht.
The club were sanctioned by European governing body UEFA in July for breaching financial regulations.
Unai Emery's men, who finished sixth in the Premier League last season, begin their campaign at home to Newcastle next weekend.