The England Under-21 midfielder had two impressive loan spells in the Championship with Charlton Athletic and Swansea City last season, where his performances earned him plenty of admirers from Premier League clubs for the new campaign.
The new Albion number 18 becomes Slaven Bilic's seventh signing of this transfer window and his second signing in as many days after the arrival of ex-Chelsea defender Branislav Ivanovic.
Ahead of his transfer to the Baggies, Gallagher put pen to paper and signed a five-year contract with Chelsea FC, where he is regarded as one of their brightest young talents. The midfielder won the Blues' Academy Player of the Year award for the 2018/19 season.
"I’m delighted to be here" - Gallagher
Speaking to West Brom's official website, the midfielder claims his games against Albion in the Championship last season played a part in wanting a move to Bilic's side.
“There was a lot of interest from the Premier League, which is where I wanted to play this season, but this is where I want to be,” he said.
“West Brom stood out to me because of the way they played last season. I played against them two or three times and I liked how they approached the game.
"I felt that it would suit me and it would be best for my development. I felt it could help me become the player I want to be and I’m pleased to get this agreement sorted.”
Midfield depth no longer an issue for Bilic's side.
The signing of Gallagher has added a new dimension to Bilic's midfield, with the tricky youngster able to play as a box-to-box midfielder as well as strongly contributing to the attacking side of the Baggies play.
Gallagher was able to showcase his attacking influence over both his loan spells at Charlton and Swansea last season - managing six goals and 11 assists in 47 total appearances.
The current Albion central midfielders don't offer such a goal threat as Gallagher yet are a lot stronger defensively, leaving Bilic with different options to choose from when looking to break down different types of opposition.
“I like to think I’m a box-to-box player but I’m also versatile,” he says. “I like to think that I can play wherever in midfield and look to create chances and try to chip in with goals too. I hope I’ve got a lot to offer.
“The challenge doesn’t worry me. I chose to go to Charlton last season knowing it would be tough but I felt I could flourish as a player in those circumstances.
“I’m willing to help the team as much as I can.
“I spoke to Slaven Bilić the other day just to see how things stood and I liked what I heard and how he wanted me to play. It all made sense to me and I’m really looking forward to getting started.”