Conor Townsend has become Albion's fifth signing of the summer after joining from Scunthorpe for a fee believed to be around £750,000.
The 25-year-old will now compete with Kieran Gibbs for the left back position after impressing last season at the League One club.
"This is another good acquisition for us," said Albion boss Darren Moore.
"He's been on our radar for quite a while and I'm delighted we have beaten two other clubs to his signature."
Townsend came through the ranks at Hull City and had loan spells at Grimsby, Chesterfield, Carlise, and Dundee United before signing permanently at Scunthorpe.
The attacking full-back has signed a three-year-contract at The Hawthorns.