The fight will still take place, and Whittaker will retain his title if he wins. However, Romero is no longer eligible to win the title.
"It's going to be a five-round fight. Everything goes on as usual," UFC president Dana White said.
Romero, 41, weighed in at 186 pounds in his first attempt Friday. The Illinois State Athletic Commission afforded him two additional hours to make the 185-pound weight limit, but Romero came up short at 185.2 pounds.
The two fighters were introduced and faced off at the ceremonial weigh-ins later Friday. Romero said the commission ended up giving him just one hour, not two, to cut to the weight limit, and he received no explanation for the shortened opportunity.
"It's very hard. We come here to fight for the title," Romero said. "And the commission makes this mistake. We want to fight for the title."
It's the second consecutive bout for which Romero (13-2) has missed weight. He was scheduled to fight Luke Rockhold for an interim title at UFC 221 in February but came in at 187.7 pounds. The bout was eventually salvaged. Romero knocked out Rockhold in the second round.
Earlier in the week, Whittaker was asked whether he would take the fight if Romero missed weight again. He jokingly responded that it's a "long flight" from his home in Australia and added, "I'm fighting somebody."