The Brazil midfielder made an announcement himself on social media yesterday afternoon, which included a video produced in partnership with the club.
United have since released a lengthy statement to pay tribute to Casemiro, who moved to Old Trafford four years ago from Real Madrid.
The club confirmed: "Casemiro will leave Manchester United this summer, upon the expiry of his contract.
"The legendary midfielder has enjoyed four seasons at the club, playing 146 games and scoring 21 goals to date.
"A serial winner, Casemiro was unveiled at Old Trafford in August 2022, and quickly became a fan favourite due to his combative nature and ability to score goals at crucial times.
"The Brazil captain played a major role in helping United to win the 2023 Carabao Cup, scoring a header in a colossal performance against Newcastle United. The 33-year-old also won the Emirates FA Cup in 2024.
"Supporters will have the opportunity to recognise Casemiro’s contributions at the final home game of the season, against Nottingham Forest."