The Egypt international, who helped Premier League newcomers City win promotion from the Championship while on a season-long loan, has signed a three-year contract after the clubs agreed an undisclosed fee.
And the 25-year-old winger is delighted to continue his working relationship with Steve Bruce, the Hull manager who brought him to England during his time in charge of Sunderland.
"I really enjoy playing for him and I look forward to doing so again now," Elmohamady told the Hull website.
"It feels great to be here permanently, especially after what happened here last season.
"It was a great year and I thoroughly enjoyed playing for this club. Everything about it was great - the players, the staff, the fans - and I am delighted to be here again.
"It's going to be great to play in the Premier League again, especially with Hull City, and I am very much looking forward to the new season.
"Moving here was an easy decision to make. The fans are fantastic and it will be enjoyable to play in front of them in the Premier League."
Elmohamady, who first arrived on Wearside on loan in the summer of 2010, made 61 appearances for Sunderland.