Koumas had already passed a medical, and the Wales international agreed terms on a four-year contract on Tuesday.
The deal represents a £3million profit for West Brom on a player who cost them £2.25million from Tranmere in 2002.
© Copyright PA Sport 2007, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: https://www.givemesport.com/writeforgms