The 33-year-old moved to Old Trafford from Real Madrid in August 2022 for a fee rising to £70million, signing a four-year deal with an option for a further season.
But the Press Association understands United decided against keeping Casemiro beyond the end of the campaign and have agreed to the player’s request to confirm his summer exit at this time.
“I will carry Manchester United with me throughout my entire life,” the Brazil international said.
“From the first day that I walked out at this beautiful stadium, I felt the passion of Old Trafford and the love that I now share with our supporters for this special club.
“It is not time to say goodbye; there are many more memories to create during the next four months.
“We still have a lot to fight for together; my complete focus will, as always, remain on giving my everything to help our club to succeed.”