Brooks (16-1) absorbed some decent shots by Jansen in the first round, but more or less dominated the rest of the way. The fight headlined Bellator 136 inside Bren Events Center.
By the third round, the challenger was bloodied and shooting desperately for takedowns. All three judges scored the 155-pound title fight comfortably for Brooks, 49-46.
"It feels amazing, especially fighting a guy like Dave," Brooks said. "I know we did a lot of talking before the fight. I think that was just emotions. When you're fighting for the title, your emotions are high. Dave showed his warrior spirit tonight. I hit him with some hard shots. I felt like I was going to break my hand. The guy is tough as hell."
Jansen (20-3) certainly waited a long time for his shot at the title -- more than two years actually. The 35-year-old earned a title shot by winning the season seven lightweight tournament in March 2013 but was forced to wait on the sidelines while Brooks and former champions Michael Chandler and Eddie Alvarez fought over the belt.
The Oregon-based challenger wasted no time getting after Brooks in the opening round. He threw a barrage of kicks at Brooks from the outside, before switching gears and closing distance on the champ with punches. Once inside the clinch, Jansen unleashed a string of knees to Brooks' midsection, which seemed to get his attention.
Jansen struggled to maintain his rhythm after the first round, however, as Brooks started to find a home for his right hand and high left head kick. The champion's confidence visibly grew as he started to put together combinations and he scored a takedown against Jansen late in the second round.
Looking to turn the tide, Jansen started hunting single leg takedowns in the third with limited success. Brooks calmly stuffed the majority of the attempts and started to score at will with the right hand. He opened a cut on Jansen's right cheek with a punch and, in the fourth round, stuck his tongue out toward the crowd as he peppered Jansen to the body near the fence.
Despite the well-rounded performance, Brooks said he could have done more.
"I think there are a lot of things I could have done better," Brooks said. "I'm always judging myself the hardest. I'm going to get back in the gym and continue trying to get better."
The 28-year-old Brooks extended his win streak to seven. He is coming off back-to-back wins against Chandler (12-3) in 2014.
Schilling falters against Carvalho
Former world kickboxing champion Joe Schilling's plan to fight for a Bellator title in 2015 took a serious hit Friday.
Schilling (2-4) suffered a split-decision loss to slight underdog Rafael Carvalho in a middleweight bout. Judges Marcos Rosales and Mike Bell each saw it for Carvalho via scores of 29-28 and 30-27, respectively. Judge Mark Smith had it for Schilling, 29-28. ESPN.com scored the fight 29-28 for Carvalho.
The 31-year-old Schilling, who still competes in kickboxing and is eyeing a professional boxing match this year as well, failed to defend several key takedowns, which cost him the fight. Carvalho (11-1) took him down multiple times in the second and third rounds to jump ahead on the scorecards.
In the first, Schilling quickly took the center of the cage and landed a handful of kicks to the body and a right hand. Carvalho moved to tie Schilling up in the clinch, which resulted in a long stalemate along the fence until referee Mike Beltran ordered them to separate.
Carvalho scored one takedown in the opening round, but Schilling executed an impressive sweep almost immediately and finished the frame on his feet.
It was tougher sledding for Schilling the rest of the way though, as he spent significant stretches of time on his back and turtled up on the canvas, eating hammerfists. He came out hot in the third round, dropping Carvalho with a right hand in the center of the cage but found himself on his back moments later.
The Brazilian middleweight improved to 2-0 in the Bellator cage. He scored a first-round knockout over Brian Rogers in his promotional debut in September.
Schilling, who has said he would like to fight for the recently launched Premier Boxing Champions, drops to 1-1 in Bellator.