The 23-year-old defender has penned a five-year contract with the Spaniards.
"I have chosen Benfica because they are one of the biggest clubs in Europe and many Argentines have succeeded here," Lopez told the club's official website.
"I am anxious to start. If I play well here I may get a chance in my country's national team."
Lopez joined Arsenal de Sarandi from Chacarita Juniors in 2010.