Seymour, 29, was released by Hull last year after crashing his car while suffering from depression, and had hoped to get back into Super League with the Tigers.
He didn’t do enough to earn a deal with Daryl Powell’s men, but now steps down to the Championship on a 12-month deal.
Whitehaven Chief executive Barry Richardson said: “We’ve signed him subject to a successful visa application. We’ve been working on it and hopefully it will only take another two to three weeks.
“We’ve two young half-backs and we felt we needed a bit of experience as well as competition for places and, when we knew Brett was available, we thought he would be an outstanding signing for us.”