The 23-year-old defender has penned a five-year deal with the Europa League finalists, after being persuaded to join by compatriot Pablo Aimar
The 23-year-old defender has signed a five-year deal for an undisclosed fee, with his contract believed to include a €35 million buy-out clause.
Lopez has frequently been overlooked for Argentina duty by coach Alejandro Sabella despite impressive performances for Arsenal, and he admitted his desire to break into the Albiceleste coach's plans was a significant factor in his decision to move.
"I have chosen Benfica because they are one of the biggest clubs in Europe and many Argentines have succeeded here," Lopez told Benfica'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 revealed he had been persuaded to join the Europa League finalists after compatriot Pablo Aimar had extolled the benefits of life in the Portuguese capital.
"He said Lisbon was beautiful and Benfica was huge and magical. He told me to enjoy my stay here to the full," Lopez added.
Lopez becomes Benfica's seventh signing of a busy summer so far, following Stefan Mitrovic, Lazar Markovic, Miralem Sulejmani, Filip Djuricic and Uros Matic – as well as central defender Steven Vitoria.
Lopez's arrival could cast further doubt over the future of Ezequiel Garay, who has been linked with a move to Manchester United this summer.