Merrin, 29, is Rhinos’ second marquee signing, after Tongan centre Konrad Hurrell, which means part of his wages will not count on the salary cap.
Reports in Australia suggested Merrin had turned his back on Rhinos to remain at NRL side Penrith Panthers, but yorkshireeveningpost.co.uk confirmed on Monday Leeds were stil hopeful of getting their man.
Merrin has played more than 200 games in the NRL, having spent seven seasons with the St George Illawarra Dragons before joining Penrith in 2016 and winning a Grand Final in 2010.
He earned the last of his seven Test caps against New Zealand last year and has made 13 State of origin appearances for New South Wales.
Rhinos boss Dave Furner was an assistant-coach with Australia during Merrin’s stint as a Kangaroo,
Merrin said: “I have done everything I possibly could in the NRL and this is a great opportunity for me to come to a great club with a fantastic culture and hopefully achieve some goals together.
“It is good to have clarity about my future, I am looking forward to coming to a new country, stepping outside of my comfort zone and being part of something special with Leeds.”
He insisted: “There is a lot I still want to achieve. I want to put my best foot forward and, when I eventually come back home, I want to be able to look back on a special period in my life.
“I have spent time with Dave Furner in camp before and enjoyed the experience. He is obviously well respected at Leeds from his time there as a player and has built an impressive coaching career in the NRL since then.”
Merrin is the fourth new face in Rhinos’ 2019 squad, alongside Hurrell and half-backs Tui Lolohea and Callum McLelland.
Director of Rugby Kevin Sinfield said: “We are delighted to have signed Trent and look forward to welcoming him to the Rhinos club.
“He is an international-class forward who was a regular last season for Penrith Panthers in the NRL.
“He has an outstanding reputation in Australia for his leadership within a group and is someone who has played for a decade at the highest level of the game.
“He will complement our current group of forwards and is another indication of the ambition of the club to move forward into a new exciting era.”
Rhinos chief executive Gary Hetherington also hailed the deal.
“This is hugely significant for not just Leeds Rhinos but the Super League competition and shows the ambition of our club to compete at the very top of the game,” he said.
“Trent was Dave Furner’s number one target and it has come as a major boost to have secured a player of his quality on a long-term contract.”
Penrith executive general manager Phil Gould had earlier indicated Merrin would be staying at Panthers.
He commented: “Trent Merrin has given our club three years of outstanding service, both on and off the field.
“Panthers played finals football in each of the three seasons he spent at the club.
“Apart from the tremendous football he has played, Trent has been instrumental in mentoring the careers of a number of our young players.
“We wish Trent well with his new challenge in the English Super League. Leeds has secured itself a very good man.”