The 20-year-old impressed during his time at Charlton Athletic and Swansea City last season, and he has also been rewarded with a new long-term deal.
The midfielder put pen-to-paper on the contract on Wednesday before heading up to the Midlands to complete his season-long loan today (Thursday).
There has been a lot of interest in the England U21 international this summer, with a dozen clubs all vying for his signature.
Gallagher looked set to completing a loan spell to Crystal Palace, having completed a medical and the clubs agreeing terms.
But the Eagles prioritised a move for a striker and when they were able to snap up Michy Batshuayi for the campaign, Gallagher’s move fell through due to rules over only allowing to loan one player from another Premier League club.
West Brom became the frontrunners to take him on for the season, however football.london understands fellow Birmingham side Aston Villa made a late attempt to get him.
But Gallagher is now set to spend the season with Slaven Bilic and at the Hawthorns to get Premier League experience.