Armstrong has agreed a three-year contract worth pounds 150,000 with the Kingston Park side after more than two weeks of negotiations. He will report to his new club for a medical and training on 2 January and is expected to be named in the side to play Wakefield on 6 January.
"It's a great relief to have signed," Armstrong said. "The contract was with my lawyer for over a fortnight. The negotiations have taken longer than I would have liked but it's a big step and we had to get the details right."