The 20-year-old, who has already played 41 times for the Lions, claimed his decision to re-sign was influenced by the men he plays with and the love he has for the club.
“I just think each week, each day, I see 47 blokes come in with the same goal, and that’s to improve,” Witherden told the club’s website.
“I love my teammates and I’ve got some of my best mates here now. I would never want to leave them.
“That was one of the main reasons I wanted to stay.”
The Brisbane defender was drafted to the side with Pick No. 23 in 2016.
“The way the Club’s going at the moment, we’re starting to see the on-field benefits of all the hard work we’ve been putting in behind the scenes over the past 24 months,” Witherden said.
“I think if we continue to work hard, we’re destined to have some form of success in the future.”
General Manager of Football David Noble claimed Witherden was a determined individual with an exciting future ahead of him.
“Alex’s commitment to extending his contract allows us to keep developing the group we drafted together a few years ago,” Noble said.
“His infectious personality and growth as an individual, along with his investment in his teammates and the Club, has been fantastic to watch.
“We have loved watching his on-field skills mature and become a significant part of our backline.”