He’s now the only fighter who can stake a claim to the lightweight title.

Nurmagomedov improved to 26-0 (10-0 in U.F.C.) in a path to the lightweight championship much more laborious than his five rounds in the ring at Brooklyn’s Barclays Center. He won 50-43 on two scorecards and 50-44 on the third.

Nurmagomedov had four fights against interim champ Tony Ferguson fall through — the latest because of a freak knee injury that led to featherweight champ Max Holloway getting the call on six days’ notice to move up in weight. Holloway was ruled medically unfit to compete on Friday, which forced U.F.C. to bump Iaquinta from an undercard bout against Paul Felder to a title fight.

Nurmagomedov was on the bus and said he had to be restrained by security to keep from fighting McGregor in the underbelly of the arena.

Nothing stopped him against Iaquinta.

The 29-year-old Nurmagomedov once wrestled a bear as a child and he had no trouble busting open a clearly overmatched Iaquinta. He used takedowns in the first, second and fifth rounds that led to repeated blows to the back of the head that cleared the way to his first U.F.C. title.

“I want to say thank you to Iaquinta for stepping up and fighting. He is a real Brooklyn gangster,” Nurmagomedov said.