John Ruiz (born January 4, 1972) is an American former professional boxer who competed from 1992 to 2010, and held the WBA heavyweight title twice between 2001 and 2005.

Though he had been undefeated since his 1st round 19 second loss against David Tua in 1996, the level of competition Ruiz had been facing was suspect and the only name he had beaten (to date) was a nearly 40-year-old Tony Tucker.

A judge agreed, but rather than face Ruiz in a bout that was seen as commercially unattractive, Lewis instead fought Michael Grant, considered to be a very worthy contender at the time, having knocked out a series of recognized "name" opponents on HBO.

After learning of this, the judge decreed that upon entering the ring against Grant on April 29, 2000, Lewis would automatically forfeit the WBA title.

Some critics still believe the decision was controversial (Ruiz was on the ground for a few minutes after Holyfield appeared to deliver a low punch to the groin).

On March 1, 2003, Ruiz was contracted to fight Roy Jones Jr., who at the time was The Ring light heavyweight champion.

On November 13 of that same year, Ruiz retained the belt with a controversial unanimous decision over Polish-American[3] Andrew Golota – among other things he suffered two knockdowns and a one-point deduction by referee Randy Neumann.

Frustrated by years of criticism from the boxing press and fans, he retired upon his second loss of the WBA title on April 30, 2005 (to James "Lights-Out" Toney).

However, after Toney failed the post-match drug test (for stanozolol, an anabolic steroid), the New York Athletic Commission suspended him from boxing in the United States for 90 days, and fined him $10,000 (U.S.).

Ruiz's long-time manager, Norman Stone, declared that they would also formally petition the WBA: after all, the 10,000 German spectators booed when the decision was announced.

Wilfried Sauerland, the manager who rescued Valuev's career from obscurity two years earlier, angrily countered that the fans had booed because Stone's in-ring behavior had agitated them.

Chagaev became the mandatory challenger for a shot at Valuev, whom he defeated on April 14, 2007, to claim the WBA heavyweight championship.

As that bout was cancelled, Ruiz stepped aside as mandatory challenger so Valuev could fight Cruiserweight Champion David Haye.

Ruiz, coming in at 226 pounds (his lightest since 2001), dominated his overmatched opponent to retain his mandatory challenge to the WBA belt.

In 2013, he opened Quietman Sports Gym in Medford, Massachusetts, offering both boxing and MMA (mixed martial arts) training to all ages.