The midfielder has put pen to paper on a two-year deal at the City Ground after four seasons in Germany, where he served as vice-captain. Before Leipzig, Schlager had spells at Wolfsburg - where he first worked under Glasner - as well as Red Bull Salzburg and Austrian side Liefering.
Reflecting on the move, Schlager pointed to Forest's history and his existing relationship with the head coach as key factors.
"It is a huge honour to join Nottingham Forest. It is a very traditional club with a big history, winning two European Cups under Brian Clough. I wanted a new challenge and to join a club with good players, and I am happy to be reunited with Oliver, who I know from my time at Wolfsburg."
He also outlined his ambitions for the coming season, with a return to European competition high on his list of priorities.
"The club has big ambitions and so do I. I want to be back in European competitions and we have a good chance. It is the best feeling to play internationally during the week, and I want to achieve that."
Schlager was an ever-present for Austria at this summer's World Cup, starting all four of his country's matches, and has now won 55 caps at senior level. He also won the Austrian Bundesliga title four times during his time with Salzburg earlier in his career.
Forest's chief football officer, George Syrianos, welcomed the experience Schlager brings to the squad.
"He's a leader, with years of experience playing at the highest level in Europe, and there is a strong feeling he has all the attributes to make an immediate impact on the team."
Schlager departs Germany having made 150 Bundesliga appearances across his time at Wolfsburg and Leipzig, scoring seven goals in the process. He lifted the DFB-Pokal with Leipzig in 2022-23 and added the DFL-Supercup the following season, on top of the Austrian Bundesliga titles won earlier at Salzburg.
His reunion with Glasner dates back to a two-year spell together at Wolfsburg between 2019 and 2021, during which Schlager established himself as a first-choice midfielder despite injury setbacks including a serious ankle problem and a cruciate ligament rupture that required surgery.
Forest will be hoping his experience can help offset the loss of key personnel elsewhere in the squad as Glasner begins reshaping the team.