Pimblett, the pre-fight favourite, showed courage but was ultimately bested by the American, who at 37 demonstrated that time has yet to catch up with him.
Despite both fighters raising their arms after the final bell, all three judges awarded the contest to Gaethje, who claimed the interim belt for a second time.
The Liverpool native forced Gaethje into a relentless contest, trading blows until the closing seconds. Gaethje responded in kind, raising his level and drawing Pimblett into a fight that recalled his best years in the octagon.
“Paddy is right, Scousers do not get knocked out,” Gaethje said afterwards. “My coach was definitely upset at me after the first round, but I just love this so much. It’s really hard to control myself sometimes.
“I knew I had to put him on his back foot. He is very dangerous and has great timing. I had to steal his momentum and confidence.”
Gaethje’s victory sets up a future meeting with the absent champion, Ilia Topuria, once he returns from a personal hiatus.
Pimblett, 31, applauded his opponent as the scores were read, accepting his first defeat in the UFC with grace.
“I wanted to be walking away with that belt,” he said. “I know how tough I am, and I don’t need to prove that to anyone. I think 48-47 was a fair scorecard.
“He caught me with a body shot in the first round, and it got me. Up to that point, I thought I was winning the round.
In defeat, Pimblett again used his platform to address mental health, urging men to speak openly about their struggles. “Two lads I know have killed themselves over the last few months,” he said. “Men, speak up. Don’t bottle your feelings up.”