The 21-year-old, whose impressive form helped the Bees reach the play-off final last season, has committed himself to Griffin Park until 2016.
"I'm absolutely delighted," he said.
"It was always Brentford I wanted to be at but it was just about getting the details sorted. I'm looking forward to repaying the faith the club has shown in me.
"There were things in the press about other clubs but all I am thinking about is Brentford.
"If I had left, after last season, it would have been a job half done in my eyes. This year is time to repay everyone, do the business and get promoted to the Championship."
Dean played for 60 minutes as the Bees sank Millwall 3-0 in a pre-season friendly on Tuesday night thanks to goals from Sam Saunders, Paul Hayes and Stuart Dallas.
"I enjoyed it," he added. "It just shows how good our squad is, turning over a Championship side, albeit in pre-season.
"It's another good result, more fitness in the bank and we'll take that on to our friendly against Celtic on Saturday."