Undisputed lightweight champion

Whitaker trudged on, winning a decision over Greg Haugen for the IBF Lightweight title on February 18, 1989, becoming the first boxer to knock Haugen down by sending him to the mat in the sixth round. He then added the vacant WBC & The Ring belts by avenging his loss to Ramirez on August 20.

Now a champion, Whitaker proceeded to dominate boxing's middle divisions over the first half of the 1990s. In 1990, he defended his Lightweight title against future champion Freddie Pendleton and Super Featherweight Champion Azumah Nelson of Ghana. On August 11, 1990, he knocked out Juan Nazario in one round to win the WBA and vacant lineal lightweight titles,^[9] becoming the first Undisputed Lightweight Champion since Roberto Durán. His highlight of 1991 was a win over Jorge Páez and a fight against European Champion Poli Díaz that ended in another win.

Whitaker vs. Chávez

Whitaker was gaining momentum and boxing experts and fans felt that he needed to win against the pound for pound best boxer in the world: Julio César Chávez. The two met in a welterweight superfight simply named "The Fight"^[11] on September 10, 1993 in San Antonio, Texas. In the eyes of many of the spectators, Whitaker outboxed the Mexican legend. While one judge did score the fight in favor of Whitaker (115-113), the other 2 judges saw an even bout (115-115), resulting in a majority draw. Sports Illustrated's post-fight edition had "ROBBED!" across the cover and they believed that Whitaker had won no less than 9 of the 12 rounds in the fight.^[12] The now-defunct “Bert Sugar's Boxing Illustrated" magazine had a heading on the cover of its post-fight edition telling readers not to buy the issue if they really believed "The Fight" was a draw.^[13]

Whitaker continued on to dominate for the next few years, defending his welterweight title against Santos Cardona on April 9, 1994 and in a rematch against McGirt on October 1, 1994.

Whitaker vs. De La Hoya

He met Oscar De La Hoya on April 12, 1997, in Las Vegas, Nevada. Whitaker, defending his WBC championship and the mythical status as the best boxer in the world, pound for pound, succeeded in making De La Hoya look bad through his crafty defense, but he was unable to mount a sufficient offense to convince the judges. Whitaker was awarded an official knockdown in the 9th round and, according to CompuBox stats, outlanded De La Hoya in overall punches and connect percentage, using the jab as his primary weapon; but De La Hoya threw and landed almost twice as many power punches and had a slightly higher power punch connect percentage than Whitaker and showed more aggression in the fight, which may have been the key factors in De La Hoya winning by a disputed unanimous decision. At the end of the fight, the judges' scores were 111-115, 110-116, 110-116. The fight was much closer than what the scorecards showed, and many observers felt that Whitaker won the fight. It was another controversial decision against Whitaker, but it wasn't seen as a blatant robbery like the Ramirez or Chavez decisions.^[18]

For his part, De La Hoya wasn't too pleased with his own performance against Whitaker and had hinted at a possible rematch to prove that he could do better against him. But Bob Arum, De La Hoya's promoter at that time, decided against it. ^[19][20][21]

Whitaker's next bout was against Russian-born fighter Andrey Pestryayev in a world title elimination fight, where the winner would earn an automatic #1 contender spot for the WBA Welterweight crown, held at the time by Ike Quartey. Whitaker originally won the fight, but the win was nullified & changed to a No Decision after he failed a post-fight drug test.^[22][23] Whitaker was suspended for six months, but the commission lifted the suspension after he agreed to random testing and his title bout with Quartey was to proceed as scheduled, however, the bout was cancelled after Whitaker tested positive for a second time.^[24]

Whitaker vs. Trinidad

On February 20, 1999, Whitaker suffered his first convincing defeat against the much bigger, much fresher Félix Trinidad, gamely taking the Puerto Rican the distance in an attempt to win Trinidad's IBF welterweight title.^[25] The fight began with both boxers displaying aggressive styles, which included excessive pushing. In the following rounds, both boxers used their jabs most of the time, with Trinidad gaining an advantage when Whitaker attempted to attack inside, eventually scoring a knockdown in round two.^[25] In the fourth, fifth and sixth rounds the fighters exchanged combinations.^[25] Later in the fight, both boxers fell to the canvas in what were ruled as "accidental slips."^[25] In the seventh round, Whitaker displayed more offense, trading power punches with Trinidad, but the champion retained control of the fight's tempo during the eight, ninth and tenth rounds.^[25] In the last round, Whitaker, with a badly swollen right eye, displayed a purely defensive stance, avoiding his opponent throughout the round while Trinidad continued on the offensive until the fight concluded. The judges gave the champion scores of 117–111, 118–109 and 118–109.^[25]

His last fight came on April 27, 2001, against journeyman Carlos Bojorquez. Whitaker, the former lightweight, entered the ring at 155 pounds. He broke his clavicle in round four and was forced to retire; at the time of the stoppage Whitaker was trailing in all the judges' scorecards by 28-29. Following this fight, Whitaker officially announced his retirement. He finished his professional career with an official record of 40-4-1 (17 knockouts).

In 2002, The Ring ranked Whitaker as the 10th Greatest Fighter of the Last 80 Years.

On December 7, 2006, Whitaker was inducted in the International Boxing Hall of Fame along with contemporaries Roberto Durán and Ricardo López. They were all elected in their first year of eligibility.