The Australia international is one of a number of high-profile overseas captures arriving in the UK ahead of 2019.
Leeds have also signed Tonga star Konrad Hurrell from Gold Coast Titans and Tui Lolohea from Wests Tigers.
Warrington Wolves snared Canberra Raiders ace Blake Austin, Huddersfield Giants have recruited Manly Sea Eagles flier Akuila Uate and St Helens have secured the likes of Lachlan Coote and Kevin Naiqama from the NRL.
It could represent a sea-change moment for Super League which, for some time, had struggled to attract the top-end quality from Australia.
New South Wales State of Origin forward Merrin, 29, admitted: “It’s definitely good that players like myself are coming.
“When I signed, I saw a clip on social media about how many players have come over.
“The quality of players that are coming to Super League – and earlier on in their careers too – it’s great to see. The quality’s only going to get better and better.
“The more quality we get, it’s definitely going to build the quality here.”
Merrin’s decision to move to the UK was aided by Dave Furner, his former Kangaroos assistant coach who has joined at the same time having left his role at South Sydney.
“I spent a bit of time with Dave with the Aussie team and he’s excellent,” he said about Furner, who won a Grand Final as a player with Leeds in 2004.
“As soon as I knew he was coaching over here, I spoke to him and he made me very excited.
“It made the transition very easy for me. Having seen the level of coaching, it’s exceptional.”
Merrin, who landed earlier this week, after leaving Penrith Panthers, is set to make his debut against Wakefield Trinity in the Festive Challenge on Boxing Day.
He has taken a look at Emerald Headingley, which is shaping up nicely on the back of a £45m rebuild, and said: “I haven’t played here before.
“But I had a look on the first day and it’s spectacular.
“It’s a big reason why I wanted to come here and to experience the crowd.
“I know the crowds love it here; I played a couple of Tests here (in England) for Australia and just feeling that vibe is something special.”