Arsenal manager Mikel Arteta hopes that the new signing will boost his team's defence and increase their chances of challenging for the Premier League title after missing out again last season.
He told the club's website: "We welcome Riccardo and his family to Arsenal. He is a great signing and gives us huge strength to reinforce our defence.
"Riccardo is a big personality and character, with specific skills which will make us stronger as we push to win major trophies. He has already shown great development in recent seasons with his performances for both Bologna and Italy, with his progression and development in the past year being really impressive."