The 22-year-old joins the Black Cats on a temporary deal until the end of the season, having made just three Championship starts for Boro so far this term.
The striker came through the ranks at Manchester United and has also spent time at Barnsley and West Ham United before joining Middlesbrough in the summer.
You up? pic.twitter.com/d1m6ybG2LS— Sunderland AFC 🔴⚪ (@SunderlandAFC) February 1, 2018
"Ash has experience at this level and in the Football League. He's played for big clubs and in big games," Sunderland boss Chris Coleman told the club's official website.
"He's a physical presence and a handful for defenders. More importantly he wants to come here, progress and fight with us."
Fletcher could make his Sunderland debut at home to Ipswich Town on Saturday.