The 31-year-old former Sheffield United and Portsmouth player joined Rovers in February, signed during Michael Appleton's brief tenure as manager, and made his debut in the shock 1-0 FA Cup triumph over Arsenal.
Williamson's new deal keeps him at Ewood Park until summer 2015.
"I came here on a short-term contract, so I'm happy that it has now been extended," he told Blackburn's official website.
"I like it here. It's a big club, there are a lot of good players, the structure is good, the ground and the following from the fans.
"Now I've signed an extension here I can establish myself and this club can push forward. As a player there is always room to improve, so hopefully I can show that this coming season."