The 25-year-old scored one goal in 36 appearances for Atalanta last season and is Boro's third signing of the close season, following the arrivals of winger Viktor Fischer and defender Bernardo Espinosa.
"Really happy with my transfer to Middlesbrough! Let's make a good season in the Premier League. Come on The Boro," De Roon said on his official Twitter account.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
(Reporting by Simon Jennings in Bengaluru; Editing by John O'Brien)
