UFC 28 was the first UFC event held under the promotion’s modern unified rules format.
UFC 28 took place on November 17, 2000, at Trump Taj Mahal in Atlantic City, New Jersey. The New Jersey State Athletic Control Board sanctioned the event using the Unified Rules of Mixed Martial Arts, which helped establish the regulatory framework later adopted widely across the sport.
The rules introduced or formalized weight classes, timed rounds, round scoring, and restrictions on fouls. They also moved the UFC away from its early image of almost unrestricted, open-weight contests. The main event featured Randy Couture defending the UFC Heavyweight Championship against Kevin Randleman.
People sometimes identify UFC 21 as the rules milestone because that event used rounds and judges’ scoring. However, UFC 28 is specifically recognized as the first UFC event conducted under the Unified Rules, making it the clearer answer to this question.