The Swede signed his new deal on the eve of victory over QPR.
Clarke said: "It is a big boost for everyone at the club, a big boost for the player himself and he deserves it.
"He has been fantastic at West Brom since he signed, has worked ever so hard for the club. He is one of those defenders who puts his body on the line, he sticks his head in, he gets his blocks in.
"He can pass from the back as well, he is a quality player, and we are all delighted he has signed a new contract and chosen to stay with us."