Hughes, 22, has put pen to paper on a five-year contract at Vicarage Road and joins for an undisclosed fee.
Former England midfielder Stewart Downing has been told he can leave Middlesbrough on a free transfer this summer, according to multiple reports.
Sunderland's quest to return to the Premier League at the first attempt will begin a day earlier than their Championship rivals.
Check out the full fixture list for the 2017-18 Championship season.
Sunderland owner Ellis Short is speaking to at least two potential buyers as his efforts to sell the club gather pace.