The deal for Elmohamady, who spent last season on loan at the Tigers, is subject to the player being granted a work permit.
"It feels great to be here permanently, especially after what happened here last season," he told the club's official website.
"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."
Elmohamady scored three goals in 41 games for Hull last term.