Bellator went to court, arguing that Jackson still had three fights left on his six-fight contract.
"We are pleased by the judge's ruling and look forward to having Rampage fighting for Bellator again soon," Bellator said in a statement Tuesday.
The UFC issued a somewhat pricklier statement saying it was surprised at the ruling "because Mr. Jackson represented to UFC on multiple occasions that he was free to negotiate and contract with UFC.
"The UFC organization is also surprised that Bellator sat on its alleged rights for months before taking action," it continued. "(The) UFC understands that Mr. Jackson is considering an emergency appeal, and UFC is also considering action to protect its rights and minimize damages regarding this matter.
"UFC 186 in Montreal, Canada, will proceed as planned and UFC is currently evaluating its fight card options."
Jackson tweeted his own response, taking a shot at Bellator with the hashtag aintworriedboutnothing.
In an earlier interview with The Canadian Press after his return to the UFC was announced, Jackson had said his management had assured him he could walk away from the contract when certain problems had not been resolved after 45 days.
"I'm extremely confident it's going to be sorted out," he said. "The UFC is the biggest show on the planet for a reason. Those guys aren't idiots. They looked at everything and they said 'Oh yeah. there won't be any problems.' That's why they signed me."
The 36-year-old Jackson is slated to take on Fabio Maldonado, ranked 13th among light-heavyweight contenders, in Montreal. Jackson (32-10) last fought for the UFC in January 2013.
The UFC website still had the Jackson-Maldonado fight on the Montreal card as of late Tuesday afternoon.
The April 25 card at the Bell Centre has had its problems.
The main event between featherweight champion T.J. Dillashaw and Renan Barao was pulled when Dillashaw was injured in training. And the original co-main event between Canadian welterweight contender Rory MacDonald and Hector Lombard was cancelled after Lombard failed a drug test.
The main event now features a flyweight title bout between champion Demetrious (Mighty Mouse) Johnson and Kyoji Horiguchi.
The Montreal card was originally slated for late last year but was postponed. A Toronto card slated for September 2013 also failed to happen when the show was shifted to Las Vegas.
Follow @NeilMDavidson on TwitterSuggest a correction