Koscheck appeared on the first season of The Ultimate Fighter as a middleweight fighter. He defeated Chris Leben via unanimous decision to advance to the semifinals, where he was defeated by Diego Sanchez via split decision. Koscheck had his first official UFC fight on April 9, 2005, at The Ultimate Finale, where he defeated Chris Sanford with a first-round KO. Koscheck followed this victory with another win at UFC Ultimate Fight Night over Pete Spratt in Koscheck’s debut at welterweight. Since then, Koscheck has amassed an additional three victories after experiencing his first career loss to Drew Fickett due to a rear naked choke. Koscheck then defeated Jonathan Goulet at UFC Fight Night 6 by submission due to strikes.

Since 2006, Koscheck has been training at the American Kickboxing Academy(AKA) in San Jose, California, to which he accredits his vastly improved kickboxing technique.

Koscheck’s fight on December 13, 2006 was against Brazilian Jiu-Jitsu black belt Jeff Joslin at UFC Fight Night 7 where he won by unanimous decision.

Koscheck was then scheduled to fight Diego Sanchez on April 7, 2007 at UFC 69. This was the second encounter between the two Welterweight fighters. The first fight – which did not count towards Koscheck’s official record due to the bout being considered an exhibition match for The Ultimate Fighter television taping – resulted in a split decision win for Sanchez. At the weigh-ins for the event, Sanchez shoved Koscheck during the traditional post-weigh-in staredown. Although Sanchez continued to shout at Koscheck, no conflict ensued. In their fight, Koscheck defeated Sanchez by unanimous decision (30-27, 30-27, 30-27). Koscheck outstruck Sanchez throughout all three rounds and only shot in for a takedown once. During his post-fight interview, Koscheck repeatedly exclaimed, “Nineteen-and-one,” in reference to Sanchez’ first mixed martial arts defeat (including the two fights from The Ultimate Fighter show). UFC President Dana White announced a few weeks after the fight that Diego Sanchez had an undiagnosed illness before and during the fight which turned out to be a serious staph infection.

On August 25, 2007, Koscheck was defeated by Georges St-Pierre at UFC 74 via unanimous decision (30–27, 29–28, 29–28). St-Pierre’s wrestling skills were displayed during the match by outwrestling Koscheck, scoring takedowns, stuffing Koscheck’s takedown attempts and maintaining top position throughout most of the fight. There was speculation before the fight that Koscheck would out-match St-Pierre on the ground due to his credentials, but St-Pierre was confident that he was a better wrestler and striker and was more well-versed in submissions than Koscheck.

On December 10, 2008, Koscheck fought Yoshiyuki Yoshida in the main event at UFC: Fight for the Troops and won by KO in the first round by landing two devastating straight right punches to Yoshida’s head. The Japanese fighter remained on the canvas, while being tended to by commission doctors and was removed from the Octagon on a stretcher as a precautionary measure. He was later treated and released from a local hospital for a concussion. Koscheck received $60,000 bonus from the UFC for Knockout of the Night.

On November 21, 2009, Koscheck defeated Anthony Johnson by rear naked choke at 4:47 in the 2nd round. In the first round, Koscheck blocked a knee aimed at his head while he had two knees on the ground. Koscheck went to the ground holding his face, prompting the referee to step in and deduct a point from Johnson. Commentators accused Koscheck of faking the impact of the knee, which seemed to miss Koscheck’s face on the replays. Koscheck has said in multiple interviews that he was reacting to an eye poke, not the knee. In round two, Koscheck poked Johnson in the eye twice leading to Johnson being given time to recover. Because of this, many fans think due to the controversy in both rounds between the fighters, it should’ve been declared a No Contest. In his post fight interview, he called out Dan Hardy, the #1 contender in the Welterweight Division.

Koscheck was expected to face Mike Pierce on January 11, 2010, at UFC Fight Night 20, but instead went on to face Anthony Johnson on November 21, 2009, at UFC 106.

In 2010, Koscheck was a coach on The Ultimate Fighter season 12. The season 12 finale aired on December 4, 2010 with the main event consisting of two fighters (Michael Johnson and Jonathan Brookins) from team Georges St-Pierre.

Koscheck defeated former UFC welterweight champion Matt Hughes on September 24, 2011 at UFC 135 via KO in the first round, replacing an injured Diego Sanchez.

Koscheck was expected to face Carlos Condit on February 4, 2012 at UFC 143. However, on December 7, 2011, it was revealed Georges St-Pierre had sustained ACL injury to his left knee and the injury would sideline him for around ten months, forcing him out of his scheduled bout with Nick Diaz. Condit and Diaz faced each other at the event with the winner crowned interim UFC Welterweight Champion, while Koscheck faced Mike Pierce at the event. He won the fight via split decision.

Koscheck then faced Johny Hendricks on May 5, 2012 at UFC on Fox 3. He lost the fight via split decision.

Koscheck was expected to face Jake Ellenberger on September 1, 2012 at UFC 151. However, Koscheck pulled out of the bout citing a back injury.

Koscheck faced returning veteran Robbie Lawler on February 23, 2013 at UFC 157. Despite being able to take down Lawler several times during the fight, Koscheck lost by TKO after being rocked by a knee to the head while shooting for a takedown, then finished with punches on the ground by Lawler at 3:57 on the 1st round.

Koscheck was expected to face Demian Maia on August 3, 2013 at UFC 163. However, Koscheck was forced out of the bout with an injury and as a result, Maia was pulled from the card as well.

Koscheck next faced Tyron Woodley at UFC 167 on November 16, 2013. He lost the fight via knockout in the first round.

Koscheck was briefly linked to a bout with Neil Magny at UFC 184 on February 28, 2015. However, the pairing was cancelled and Koscheck instead faced Jake Ellenberger at the event. Koscheck lost the fight via submission in the second round.

Just 21 days after his loss to Ellenberger, Koscheck replaced an injured Ben Saunders to face Erick Silva on March 21, 2015 at UFC Fight Night 62. Koscheck lost the fight by submission in the first round.

On June 26, 2015, it was announced that Koscheck signed a multi-year deal with Bellator MMA. Koscheck was set to make his debut against Fellow welterweight Matthew Secor on January 29, 2016 in his home town of Fresno, California and was set to also be the main event.

It was announced on January 31, 2017, that Koscheck’s previously delayed Bellator debut would take place at Bellator 172 on February 18, 2017 against Mauricio Alonso. He lost the fight via TKO in the first round.

Koscheck officially announced his retirement from MMA on June 28, 2018.