Anna Mariana Casemiro has been in a relationship with the Red Devils star since his breakthrough at Sao Paulo in 2011.
The pair can be regularly seen enjoying their new life in Manchester, after Casemiro joined the Premier League in a £70million deal from Real Madrid last summer.
However, Brazilian outlet Leo Dias alleged Casemiro had previously been unfaithful to his wife, who he has two children with.
In a shock allegation, the report claims he had an affair with a Barcelona-based model and former Lindt chocolate assistant for five years.
The outlet claims, while they have not spoken since 2022, Casemiro even tried to persuade her to quit her job.
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His wife, Anna, has since hit out at the rumours and defended her partner in a post on Instagram.
She wrote: "Oh man, really? Best to investigate your sources first, I just look around and know it's not him."
Casemiro married his wife in 2014 and have a seven-year-old daughter Sara, and two-year-old son, Caio, together.
The midfielder is yet to comment on the speculation, and is still recovering from his side's 2-1 FA Cup final defeat to Manchester City on Saturday.
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Casemiro recently thanked supporters for treating him well since his arrival at Old Trafford, and insisted more is yet to come.
He wrote: "Thank you all for this first great season and above all for your love towards my family and towards me.
"I feel at home. I read you and I hear everyone say that Manchester United is back, but this is only the beginning of our journey.
"The greatness of this club and this shield must force us to demand more of ourselves. Much more.
"Here defeat is not acceptable and the glory days must return. For that I have come. Come on United!!"