Nicholas Robert Diaz (born August 2, 1983) is an American professional mixed martial arts fighter. He is a former WEC and IFC Welterweight champion. He has also competed in the Ultimate Fighting Championship, Pride Fighting Championships, and Elite Xtreme Combat before it ceased operations. Diaz was promoted to black belt in Brazilian Jiu-Jitsu by Cesar Gracie on May 8, 2007.

After years of practicing various combats sports and martial arts, Diaz became a professional mixed martial arts fighter in 2001 just after his 18th birthday and won his first fight, submitting Mike Wick with a triangle choke at IFC Warriors Challenge 15. Diaz would become a champion in just his second professional fight, decisioning Chris Lytle for the IFC United States Welterweight title in July 2002 at IFC Warriors Challenge 17.

Diaz was then invited to participate in Ultimate Athlete's King of the Mountain, a single-night tournament that took place two months later. He won his first two fights but eventually lost in the finals to Jeremy Jackson by TKO. Diaz would fight in Warriors Quest and Shooto against Harris "Hitman" Sarmiento and Kuniyoshi Hironaka respectively before winning the WEC's Welterweight Title in 2003 at WEC 6, submitting Joe Hurley with a kimura.

Diaz returned to defend his IFC Welterweight belt against the man that defeated him one year earlier, Jeremy Jackson at IFC Warriors Challenge 18. This time around it was Diaz who was able to get the win, winning by TKO in the first round. Taking notice of his success, the UFC signed Diaz over the summer and he made his debut at UFC 44, completing the trilogy against Jeremy Jackson and submitting him with an armbar in the last round of the back and forth fight that Diaz appeared to be winning on the scorecards.

Diaz returned to the Octagon at UFC 47, set to take on highly-touted Welterweight Robbie Lawler. Lawler was a heavy favorite coming into the fight but it was Diaz who took the offensive, chasing Lawler around the cage for the majority of the bout. It was in the second round where Diaz taunted Lawler in an attempt to get him to come after him. The move worked as Lawler swung wildly at him and missed, allowing Diaz to connect with a right hook that knocked out Lawler.

Diaz was then matched up with Karo Parisyan at UFC 49, but ended up losing a hard-fought split decision. He rebounded with wins over Drew Fickett at UFC 51 and Koji Oishi at UFC 53 before losing for the second time in the UFC at the hands of The Ultimate Fighter Middleweight winner Diego Sanchez at The Ultimate Fighter 2 finale.

The two came into the fight with a good amount of bad blood between them. They reportedly exchanged heated e-mails before and after the official signing of the fight, which was spurred on by Diaz's public opinion that Sanchez and the rest of The Ultimate Fighter participants were being given an easier path to the top compared to the rest of the fighters in the UFC.

Things went to another level in the locker room before the fight when Diaz taunted Sanchez and threw a shoe at him. Diaz was obviously confident coming into the bout but was unable to achieve success in the match, ultimately losing to Sanchez by unanimous decision. Diaz complained of a "greasy substance" on Diego's arms that impeded Diaz's takedowns. While no illegal lubricant was ever investigated, Sanchez's hair was considerably wet-looking throughout the fight, suggesting that his hair product may have leaked onto his shoulders. During the televised post-fight interview in the ring, Diaz continued the controversy by declaring that he respected Sanchez fighting ability but did not think he deserved to be there, despite Sanchez's win.

Diaz's next fight was against Joe Riggs at UFC 57. Similar to his treatment of Sanchez, Diaz made sure that he taunted his opponent plenty before their fight starting at the official press conference at the event in which Diaz confronted Riggs and the two exchanged words. They continued their conversation at the official weigh-ins in which both fighters had to be separated by UFC president Dana White and other officials present.

Riggs prevailed in a hard-fought battle, winning by unanimous decision and giving Diaz his second straight loss. After the fight, the two were taken to the hospital for observation and post-fight tests. It was there that Diaz confronted Riggs and proceeded to punch him in the face, knocking Riggs to his knees. Riggs then tackled Diaz into a wall and started to knee him when the police arrived and quickly rushed in to separate the two. Neither party, nor the police, decided to press charges.

Diaz lost his third consecutive match in a unanimous decision to Sean Sherk at UFC 59. The UFC subsequently terminated his contract and released him during the spring of 2006.

Diaz returned to his hometown of Stockton and participated in the International Cage Fighting Organization's inaugural event, defeating Ray Steinbess by unanimous decision. Diaz was slated to fight in his hometown again against John Alessio when he received an unexpected call the night before the fight from the UFC asking to fill in for an ill Thiago Alves at UFC 62.

After Alessio pulled out, Diaz took advantage of the opportunity, submitting Josh Neer in the third round. Seemingly back in the UFC, Diaz stopped newcomer Gleison Tibau with strikes in his next fight at UFC 65. However even after the win and the reassurance that the UFC would give him another fight, Diaz decided to sign with the Gracie Fighting Championships and leave the UFC. GFC had Diaz scheduled to fight Thomas Denny in January 2007, but due to poor ticket sales was ultimately cancelled.

Diaz made his debut for EliteXC in Hawaii on September 15, 2007 on Showtime. He won a hard fought split decision over his opponent Mike Aina.

MMA Weekly reported that Nick Diaz signed a two year deal with EliteXC, and most recently fought KJ Noons for the vacant 160 pound title. Diaz lost via doctor stoppage due to multiple cuts on his forehead. A disappointed Diaz left the arena immediately and on his way out he flipped off the doctor from the arena entrance and shoved the camera out of his face. Ironically, he stated in pre-fight interviews that the only way he was going to lose this fight was from a cut above his eye.

Following a tough win over opponent Muhsin Corbbrey at EliteXC's "Return of the KING" event in Honolulu, Hawaii , an altercation between Nick and his brother Nate broke out with KJ Noons and his corner. EliteXC executives asked Diaz to come to the ring and make a statement about a possible title rematch with Noons. Noons, a Native of Hawaii, asked the crowd their opinion, resulting in an echo of boos for Diaz. After Nick spoke over the microphone to Noons, saying, "don't be scared homey,"Carl Noons, KJ's father, lunged at him prompting Nate Diaz to throw a water bottle at the elder Noons. The Diaz brothers were quickly escorted out of the cage by a group of security staff. During interviews the next day Nick claimed Carl was intoxicated and overreacting. Meanwhile Carl claimed his incident was prompted by the thrown bottle. Video replay of the event showed Nate throwing the bottle after Carl jumped at Nick. No one was injured as the Diaz brothers left the arena entrance "throwing the bird" to the fans and Noons' corner.

Nick went on to face well-traveled veteran Thomas "Wildman" Denny at the EliteXC Event "Unfinished Business" in Nick's hometown of Stockton , CA. After a sluggish start to the fight, Diaz found his range and punished Denny during the latter half of the First Round. Diaz continued to keep the fight standing and maintained a high-paced tempo for the remainder of the fight, earning an impressive TKO victory over the veteran Denny :30 into Round Two.

Diaz was rumored to have a rematch against the current EliteXC Lightweight Champion KJ Noons, airing on CBS October 4th 2008. However, KJ Noons and his camp refused to accept the match. Mark Dion, KJ's manager was quoted as saying "As far as Nick Diaz, he's not the No. 1 contender out there.".

Diaz was scheduled to face Eddie Alvarez for the EliteXC 160 pound title on November 8, 2008 before the company ceased operations.

Diaz was victorious in Dream 3, defeating Katsuya Inoue by corner stoppage (TKO) in the first round.

With Strikeforce's acquisition of ProElite's assets, CEO Scott Coker has announced that Nick will face off against MMA legend Frank Shamrock at a catchweight of 179lbs, at their upcoming event Strikeforce: Shamrock vs. Diaz in San Jose on April, 11th at the HP Pavilion.