The 25-year-old, who had a loan spell at La Romareda last year before switching to Portugal from Atletico Madrid, has signed a five-year contract with the Primera Division club.
"I come with high hopes about the new project that is brewing at Real Zaragoza for next season," Roberto told the club's website.
"Zaragoza is my home. I am proud to come back here. I know many of my team-mates and I feel very comfortable working with young players of such quality."