Ellis, 21, is a graduate of the St Helens academy, and was taken to Leigh Sports Village by former Saints coach Ian Millward.
He will see out the rest of 2011 with the Centurions, before returning to Super League with the Airlie Birds from next season.
Ellis said: “They are a massive club with loads of potential. They are a big Super League club and I can’t wait.
“I feel this is an opportunity to show some people exactly what I can do and prove to a few people how good I can be with help from people like Rich and even Longy if he is to be offered a new deal next season.
“The move to Leigh has done me the world of good. I have loved every minute of it, but this was a too good an opportunity to turn down.”
Ellis has caught the eye with a number of fine performances this season, which has helped Leigh to the top of the Championship.
Hull coach Richard Agar added: “Jamie is a skilful young player with an outstanding kicking game and a lot of potential in front of him.
“He has made great strides and playing regular first-team rugby at Leigh under Ian Millward has really helped his game and if he can continue his development we are confident he can turn into a first-class Super League half back.”