Medel started all three games for Chile at last year's World Cup and Boca vice president Jose Beraldi has revealed they have reluctantly allowed him to leave.
"We have accepted to transfer him to Sevilla because the player said to us that he wanted to leave as it was an important opportunity for him playing football in Europe," he told Radio Belgrano.
"We talked it over with the coach and he finally accepted."
Medel, meanwhile, hopes Boca fans can understand his decision to make the move to Europe.
"I hope that the officials of Boca and the supporters understand me," he added on Radio La Red. "Sevilla have been following me for many months and they have given me a very important offer for me and my family.
"I wanted to be transferred because I don't know if I will have such a chance in the future."