The League Leaders’ Shield winners have secured the scrum-half’s arrival on a three-year deal from Huddersfield Giants.
Ellis, who played for Castleford from 2012 to 2014, had been on a successful season-long loan at Hull KR, helping them earn promotion back to Super League.
Still under contract with Huddersfield but behind the likes of Danny Brough, Lee Gaskell and Jordan Rankin, he was envisaging another loan move.
However, Daryl Powell has moved to bring back the ex-Hull FC player who played under him in the 2014 Challenge Cup final.
Ellis, 28, said: “I’m over the moon. As soon as I found out they wanted me back I snapped their hand off. It was a bit of a surprise; I’d spoken to Powelly during the year and he said he was keen to get me back but nothing really came of it. I genuinely didn’t think I’d get the chance again. I thought I’d be staying at Hull KR but obviously with Zak (Hardaker), this chance has now come up.”
England full-back Hardaker is expected to be banned after testing positive for cocaine so Powell has needed to strengthen. With Man of Steel Luke Gale at No7, stand-off Benny Roberts could switch to full-back to accommodate Ellis, who said: “I wanted to stay with Hull KR initially.
“I loved it there, have a lot of mates there and they are a bit like Cas – a really good, family-orientated club with a great fanbase.
“But then the chance to go back to Cas came up and after the success they’ve had it was too good to turn down. Hopefully they can go one better next year, win the Grand Final and I can be part of it.
“I definitely feel I’ve come back a better player since the last time I was here; I’m more experienced, more mature, have learned more about the game spending this last year with Tim Sheens.”
Powell said: “I am confident Jamie will be outstanding for us alongside Luke Gale and give us a great combination at half-back. I wanted to retain him when he first left. He’s many attributes which will add to our armoury.”
Meanwhile, Newcastle will host a fourth successive Magic Weekend – with Castleford playing champions Leeds Rhinos in a repeat of this year’s Grand Final.
Saturday May 19: Widnes v St Helens (3pm), Wigan v Warrington (5.15pm), Castleford v Leeds (7.30pm)
Sunday May 20: Salford v Catalans (1pm), Wakefield Trinity v Huddersfield (3.15pm), Hull KR v Hull FC (5.30pm).