Holgate has joined the Baggies on-loan for the season.
“I’m over the moon," he said. "When I was here before I absolutely loved it. Looking back, playing at the Albion was the most fun I’ve had in my career so far.
“There have been a few times I’ve tried to come back and it’s not happened for whatever reason, but to finally be able to get the move done this time around is great.
“It’s all happened so fast, obviously deadline day is always so hectic. I spoke with Ian Pearce back and forth over the day to get this move over the line, and I’m excited to get going under the manager.
“I’m buzzing to get going in front of the fans, too. Last time we came so close to promotion, and it was a great squad to be a part of. Hopefully this time we can go one step further.
“I think if you look the squad now, there’s obviously still a few boys from the team that I was in, and with the squad now I think we’re more than capable of having a serious go at it again, and I’m excited to be a part of that.”
Head Coach Carlos Corberán said: "Mason is another player our fans know well from his previous spell at the club.
“He has exceptional experience for a player of his age and his quality and maturity will benefit us across the season.
“He further strengthens our defensive options having played many minutes both at right-back and centre-back.”