Ellington handed in a transfer request last January after becoming disillusioned at a lack of first-team opportunities but saw a potential return to Wigan fall through unexpectedly.
"We are getting an excellent fee for his services - this deal is right for both the club and for Nathan," claimed Albion boss Tony Mowbay.
"Nathan became frustrated at the lack of starts he was getting in the team and expressed a desire to go and play regularly elsewhere.
"It was well documented during the January window that Nathan wanted to leave the football club.
"But, when a transfer didn't materialise, all credit to him for getting his head down for the remainder of last season.
"Another opportunity has arisen for him to leave and Nathan expressed a desire to take up that opportunity."