23-year-old Ratu is a Fijian international and has previously played for Halifax during a loan spell from Leeds Rhinos.
Halifax head coach Matt Calland is delighted to have added Ratu to his squad and is confident that he will prove to be a good addition to the squad.
“Mike’s got all the tools to be a top centre, not just at this level but at Super League level too,” he said.
“He’s never really fulfilled his potential, and I know he was really disappointed with how things turned out at Hull KR.
“For Mike, it’s about getting him in the physical condition to make the most of his ability. If we can do that, I’m sure he’ll be a great signing for us.”
Calland also confirmed that BARLA international full back Ryan Fieldhouse has joined the club on trial.
Fieldhouse currently plays for National Conference League outfit Leigh East and is the son of former Halifax prop John.
“We’ve lost Shad Royston and Miles is currently the only specialist full back at the club,” said Calland.
“Ryan is the best amateur full back in the country however you judge it and we’re going to take a look at him.
“His dad obviously has strong connections with the club so we’ll see what happens. He’s trained with us this week and looked pretty sharp so we’ll see how things go.”
Both players could feature in the pre-season friendly against Batley Bulldogs on Sunday, January 16.