In a cagey, tactical contest that lacked the excitement many fans anticipated, Nigeria-born New Zealander Adesanya, 30, peppered the 42-year-old Cuban with powerful leg kicks to take victory on the scorecards.
The bout started almost at a complete standstill, with Romero static in the middle of the octagon, holding a high guard.
The Sydney 2000 freestyle wrestling silver medallist’s bizarre style clearly puzzled Adesanya, who struggled to find any openings through the first two rounds.
Adesanya found more rhythm in the second half of the fight, however, scoring consistently with powerful kicks to Romero’s lead leg.
Romero’s attacks were few and far between, and the judges’ scorecards read 48-47, 48-47, 49-46 as Adesanya negotiated his first title defence since winning the belt against Australian Robert Whittaker in Melbourne last year.
His performance did little to satisfy the crowd, who booed for long spells of the fight.
With fans jeering his post-fight interview, Adesanya said: “It was a hard fight but, cliche, I did what I had to do. I picked him apart. The legs don’t lie.
“He plays the game in lulls. He tries to get you into a false sense of security. My coaches said I need 25 minutes of sharpness, but it’s hard to engage with someone who doesn’t want to dance. I touched him up. And still.”
Asked whether he had any concerns about the judges potentially scoring the fight for Romero, Adesanya said: “As long as they didn’t listen to the crowd, I was fine.”