The winger is now committed to the club until the summer of 2017, ending any immediate threat of a transfer elsewhere.
Gus Poyet recently claimed Buckley is a better prospect than Crystal Palace's coveted Wilfried Zaha, and the Albion manager said: "We are absolutely delighted to have agreed this new deal with Will.
"He has been our match winner many times, and has been one of our key players since he arrived. He has earned this new deal with his performances on the pitch."
Buckley, who joined from Watford at the start of the 2011/12 season, told the club website: "I'm delighted. The club and Gus decided to offer me a new contract a few weeks ago and we got it sorted pretty quickly.
"I'm happy and that's why I've signed another contract here. It didn't take long for me to think about. I just can't wait to get back out there on the pitch again now."