The highly-rated Queensland star has been released from the final year of his contract with Melbourne Storm and will join Hull on a three-year contract next season.
The 27-year-old said: "This is an exciting move for me and I am delighted to be joining a top club like Hull and one with so much ambition as well as history.
"The club have tasted success in recent years and they are obviously hungry for more. Hopefully, I can help them towards those goals."
Agar, who was handed a two-and-a-half-year contract at the KC Stadium, said: "He's a winner, who goes through the front door and takes people with him.
"Michael is a world-class player who will add steel, aggression and leadership to our pack."