Johnson, 25, joined the Black and Whites ahead of the 2013 season after a spell with Wakefield, but he struggled to maintain a first-team place at the KC Stadium.
The former St Helens prop made 21 appearances overall for Hull and the club’s owner Adam Pearson said, “Paul has left the club and we wish him well for the future. He is a great lad and did a good job for us but going forward we have identified a number of players that are expected to go past him in the side.
“Chris Green has just been named in the Knights squad alongside Liam Watts and we think he has a very strong future ahead of him, whilst another Hull boy in Josh Bowden continues to develop and deserves his chance next season.
“We need to be looking after these young players and that may involve moving on some fringe players, whilst at the same time we want some salary cap space free with a view to potential moves following the upcoming World Cup.”