Lopez signed a five-year deal with a 35-million-euro buyout clause, the club added in a statement.
The Argentine strengthens Benfica's back four, which boasts experienced Brazilian Luisao and highly-rated Ezequiel Garay as the regular centre back duo.
"Since I was 20 years old I've player over 100 matches at Arsenal de Sarandi and earned experience. That helped me get transferred to one of the best clubs in Europe," Lopez told Benfica's website.
Benfica ended the season second in the league, pipped to the title by rivals Porto.
They were also runners-up in the domestic cup and will take part in the Champions League next season.