Voted into the Championship Team of the Season at the end of the 2020/21 campaign, the 26-year-old has joined the Baggies on a free transfer after his contract at Oakwell expired.
His arrival sees new boss Valerien Ismael reunited with his former skipper at Barnsley.
Mowatt turned down the down 'the most lucrative deal ever offered to a Barnsley player' to join the Baggies.
The former Leeds man was the subject of a failed seven-figure bid from Millwall last January and was also on the wanted list of several other Championship clubs.
But Albion's status as one of the promotion favourites together with the chance to work under Ismael again saw him enticed to The Hawthorns.
"Last year Alex put his performances on the next level,” Ismael said.
“He is an outstanding midfield player with outstanding qualities and mentality. He scores goals outside the box and on set-pieces he is very dangerous.
“He is a hard worker with the right mentality and a lot of energy. The way Alex plays will suit the West Brom mentality.
“He is a leader as well. He is still a young guy but he has a lot of experience. He has hunger and he is the exact profile of player that suits West Bromwich Albion. Alex is the first signing and the first step.
“Signing Alex is important to help the team understand the principles of our play.
"It will give the guys more confidence and understanding of how we want to play. It will be easier for them to follow him."