Casemiro will leave Manchester United at the end of the season upon the expiry of his current deal at Old Trafford.
The announcement of his departure brings an end to a four-year stay at Old Trafford, which has seen him score in a League Cup final victory in 2023 and reach two FA Cup finals, winning the competition in 2024.
Casemiro was also part of the United side that reached the Europa League final in May of last season.
The Brazilian midfielder signed for the club in August 2022 and United had the option to trigger a 12-month extension at the end of the season.
However, Sky Sports News understands United have decided not to take up that option nor renew his contract and it was a decision made well in advance of Thursday's announcement as they look to overhaul their midfield in the near future.
The decision also gives Casemiro clarity for the rest of the season as he also prepares to represent Brazil in the World Cup in the summer.
Sky Sports News previously reported in the autumn that Casemiro is one of the highest earners in the squad and it was therefore unlikely United would renew his contract, as the freed-up wage capacity would allow more space for future investment in the midfield area.
"I will carry Manchester United with me throughout my entire life," Casemiro said in a statement.
"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."
Looking ahead to the summer, as Sky Sports News has previously reported, United have an interest in the likes of Carlos Baleba, Elliot Anderson and Adam Wharton.
Casemiro enjoying Old Trafford renaissance as he prepares to bow out in style
Sky Sports' William Bitibiri:
"Leave the football before the football leaves you. The football has left him." The words of Jamie Carragher aimed at Casemiro after the Brazilian's disastrous performance in a harrowing 4-0 defeat to Crystal Palace. Casemiro was all over the place, his legs had gone and as had his confidence.
There was a feeling that the midfielder United had splashed £70m on in 2022 was no longer fit for purpose. Now, as he approaches his Old Trafford swansong, he leaves with his reputation fully intact.
Some eyebrows were raised when a 30-year-old five-time Champions League winner traded the Spanish capital for a spot in Erik ten Hag's Manchester United.
However, a stellar first season that saw United finish third was some justification as a 'quick fix' appeared a stroke of real genius. Naturally on the wane, tougher seasons and poorer performances would follow but his importance would remain a fixture.
Leaning on his experience, the likes of Ruben Amorim grew to rely on Casemiro in midfield after initially opting against fielding the veteran. Important goals like the one that helped United end a six-year trophy drought in the form of a Carabao Cup continued as Casemiro would prove clutch across various periods.
Now Michael Carrick, a world-class midfielder himself during his playing career, has also recognised his importance as Casemiro started and impressed in his derby day bow at Old Trafford.
Success is relative, especially in regards to what Manchester United now compared to the Real Madrid side Casemiro left. However, like Raphael Varane before him who made the same switch, Casemiro's significance in a side battling through transition will be a point in his career he looks back on fondly.