Barcelona Lionel Messi and his father are being investigated in Spain for allegedly defrauding the state of more than 4m euros [£3.4m].
The 25-year-old Argentina forward and his father, Jorge Horacio, are suspected of filing fraudulent tax returns for 2007-09, officials say.
The World Player of the Year expressed his surprise, denying the allegation.
His Barcelona net salary is said to be some 16m euros a year, making him one of the world's highest-paid athletes.
He has also signed a number of multimillion-dollar endorsements with major sponsors around the globe.
He wrote on his Facebook page: "We have never committed any infringement. We have always fulfilled all our tax obligations, following the advices of our tax consultants who will take care of clarifying this situation."
Argentina's media also appeared puzzled by Wednesday's development, with the Clarin newspaper describing it as "unexpected news".
Barcelona FC have not commented on the allegations.