The Wales prop joins Championship club Leigh Centurions on a two-year contract having made just seven appearances off the bench for Castleford this term.
Patrick, 28, initially joined the West Yorkshire club on loan from Huddersfield Giants last term before signing a permanent deal.
But the explosive forward has struggled and played little part in their League Leaders’ Shield success while not being in consideration for the Grand Final defeat to Leeds Rhinos.
“Larne has left the club this week and it’s a decision we have come to together to benefit the club and player,” said Castleford chief executive Steve Gill.
“Larne has an opportunity to play first-team on a regular basis in the Championship. We wish him well for the next chapter of his career.”
Relegated Leigh are hoping to make a swift return to the top-flight and head coach Neil Jukes said: “Larne is a big, powerful, explosive middle who will really add to the dynamics of the team.
“He comes with a wealth of experience and good values inherited from playing for some elite clubs in Huddersfield, Wigan and Castleford.
“Larne will be a really good asset to us as we try and achieve our goal of regaining our Super League place.”
Patrick, who was raised in Castleford but started out in Bradford Bulls’ academy alongside Sam Burgess, helped the Giants to the 2013 League Leaders’ Shield.
He played in the Grand Final two years later while on a season-long loan with Wigan but they lost out to Leeds.
Patrick made his Test debut for Wales in the 2013 World Cup but was deemed unavailable for this year’s event.
He was sidelined for nine months following two major operations on his wrist, admitting struggling both mentally and physically before his playing return in June.