The midfielder moves for an undisclosed fee after the Ligue 2 side triggered an option in the agreement that saw the Frenchman spend the 2024-25 season on loan at the Stade Geoffroy-Guichard.
Sunderland chief Kristjaan Speakman told the club's website: “Pierre moved last summer to play regularly and continue his development. He achieved this, and although it was a challenging season for Saint-Étienne, their decision to exercise this option indicates the level of his performances.
"Pierre joined us without any senior experience – he believed in Sunderland, and we believed in him. We are proud of the role we have played in his career, and he is rightly held in high esteem by our supporters.
"We all wish him well in the next chapter of his career."