At the time of the UFC's inception in 1993, Mixed martial arts was not sanctioned in the United States, and did not include weight classes. Instead of the traditional championship model, the UFC held tournaments with the winner receiving a permanent appellation. In response to criticism from Senator John McCain that saw the loss of its television deal and the banning of the sport in thirty-six states, the UFC increased its cooperation with state athletic commissions and introduced weight classes in 1997, starting with UFC 12, and began introducing weight-specific titles.

The original codification for weight classes introduced only two divisions: heavyweight, which grouped together all competitors above 200 pounds (91 kg), and lightweight, which encompassed all competitors 199 pounds (90 kg) and under. At UFC 14 the lightweight division would be renamed to middleweight, though it would still encompass all fighters 199 pounds (90 kg) and under. The lightweight moniker would later return at UFC 16 with a new division consisting of those competitors 170 pounds (77 kg) and under. Two years later a fourth weight class, the bantamweight division, arrived at UFC 26 and included all fighters 155 pounds (70 kg) and under.

In 2000, the New Jersey State Athletic Control Board completely took over MMA regulation in its home state and developed new rules and weight classes that eventually became the de facto rule set for all mixed martial arts. The UFC realigned their weight classes to comply with these new regulations in 2001, beginning with UFC 31. At the time, this brought the total number of active divisions in the UFC to five: lightweight, welterweight, middleweight, light heavyweight, and heavyweight.

It would be nearly ten years before the UFC would expand their divisional offerings to include any of the lower weight classes. The first additions came in late 2010 when the UFC merged with their sister organization World Extreme Cagefighting (WEC). Due to the WEC's focus on lighter weight fighters, this merger necessitated the addition of both the featherweight and bantamweight divisions to the UFC, starting with The Ultimate Fighter season 12 finale. In early 2012 the UFC decided they would delve even further into the lower weight classes when they announced the introduction of the flyweight division to their ranks, beginning with UFC on FX: Alves vs. Kampmann.

In November 2012, as a result of the forthcoming dissolution of their sister organization Strikeforce, the UFC announced they would be adding female fighters to their roster for the first time in the promotion's history. Initially, only the women's bantamweight division was brought over, with the division's premiere bout taking place at UFC 157. A little over a year later, the UFC announced they would be expanding their weight classes for female fighters with the addition of a women's strawweight division, the first bout took place at UFC Fight Night: Cowboy vs. Miller. In late 2016, a featherweight division was introduced for the women with the first bout to be for the inaugural championship at UFC 208 on February 11, 2017. In that same year the UFC announced the Women's Flyweight division would officially be added, with the winner of the 26th season of The Ultimate Fighter to be named the inaugural champion.

206 to 265 lb (93 to 120 kg)
The UFC Superfight Championship was unified with the UFC 11 Tournament Championship to determine the inaugural UFC Heavyweight Champion on February 7, 1997, when Mark Coleman defeated Dan Severn at UFC 12.

186 to 205 lb (84 to 93 kg)
The Light Heavyweight Championship was known as the Middleweight Championship prior to UFC 31 (May 4, 2001). The Pride World Middleweight Championship (205.03 lb) was unified with the UFC Light Heavyweight Championship on September 8, 2007, when Quinton Jackson defeated Dan Henderson at UFC 75.

171 to 185 lb (78 to 84 kg)
The Pride World Welterweight Championship (182.98 lb) was unified with the UFC Middleweight Championship on March 1, 2008, when Anderson Silva defeated Dan Henderson at UFC 82.