Reijnders, a Netherlands international, has agreed to a five-year deal at the Etihad Stadium and could debut at the Club World Cup in the United States.
Reijnders, who scored 15 goals last season and was named the best midfielder in Serie A, aims to enhance his goal-scoring record.
Reijnders expressed his excitement about joining Manchester City, highlighting the team's world-class status, coach Pep Guardiola, and the opportunity to play in the Premier League.
Director of football Hugo Viana noted that Tijjani Reijnders brings energy, composure, and creativity to the midfield.