Manuel Benitez, 79, says Manuel Diaz, 47, is not his offspring.
But a DNA paternity test has come back 99.9% positive, a Spanish court heard.
The dispute between the two men, who share the show name "El Cordobes", has been ongoing for decades. Mr Diaz says he has no financial interest but wants the relationship to be recognised.
His lawyer says the court case is now a mere formality.
The court, in southern Cordoba, agreed to consider the case after Mr Diaz presented DNA tests taken from a restaurant napkin used by Mr Benitez and picked up by a private detective.
The court then conducted its own DNA tests on blood taken from both parties, and the results came back with the highest possible positive rating.
"I want my children to have a grandfather," Mr Diaz told Hola magazine (in Spanish).
Mr Benitez was not present at the hearing.
He was one of Spain's most famous bullfighters in the 1960s and also starred in several films. He reportedly met Mr Diaz's mother when she was working as a maid in Madrid.
He has five other children - one of whom, Julio, is also a bullfighter and is also known as "El Cordobes".
A Spanish bullfighter has won a key stage in his battle to get another celebrated bullfighter to recognise him as his son.