The Northern Ireland international joined the Bees from IFA Premiership outfit Crusaders in August 2012 and made 10 appearances during his first season at the club.
Dallas played five times at the start of this term before joining Northampton on loan, where he will stay until January.
"We are very happy to secure Stuart Dallas. Stuart has made steady progress since he joined us. He has made the step from non-League football into the professional game very well," Brentford manager Uwe Rosler told the club's official website.
"He is now at another stage of his development, getting regular league football at Northampton.
"Stuart has proved himself to me; I am very pleased with his attitude and his willingness to work hard to get better. He deserves a new contract."
- Sports & Recreation
- Stuart Dallas