The Austrian joins Forest after leaving Crystal Palace and becomes the club's fifth coach in less than a year.
"I'm delighted to join Nottingham Forest as head coach," Glasner said.
From my very first conversations with the owner and the leadership team, it was evident to me that they have a clear vision for this football club and complete trust and belief in me and my staff to build a strong future together over the long term.
"That trust and shared commitment, together with the potential that I see within the squad, were key factors for me and I am excited about what we can achieve together."
Glasner guided Palace to a first-ever trophy in 2025 by beating Manchester City in the FA Cup final and then won the Community Shield and Conference League the following season.
However, he announced he would be leaving the Eagles at the end of the last campaign after disputes with the club's ownership over transfers.
"Nottingham Forest is a club with incredible prestige and history, a two-time European Champion with one of the most passionate fan bases in football," Glasner added.
"Our aim is to build a team that can help take the club to the next level in the years ahead and that our supporters can be proud of.
"My immediate focus is on meeting the players and staff and getting to work as we begin pre-season. I'm excited for the future and will work tirelessly to represent this great club with pride and to bring success on the pitch. I can't wait to get started."
Glasner replaces Pereira, who was informed by email that his services would no longer be required after he had led Forest to Premier League safety.
Forest said the Portuguese had left his role "following the club's decision to exercise a mutual break clause in his contract".