Rhodes' seventh-round stoppage of Bryan won him outright ownership of the trophy in just 90 days five days faster than the previous mark set by the lightweight Michael Ayers in 1995.
Until Rhodes, 20, finally caught up with Bryan and the referee, Roy Francis, stopped the action, it was not one of his better performances by the youngest British champion for 57 years.
The Sheffield fighter said after handing his belt to his mother, Tina: "Del Bryan had a heart like a lion and I can't believe he took some of those shots."
Bryan frustrated Rhodes until the fifth when he was floored by a right uppercut. The challenger's left eye was also cut in that session, and a left hook in the seventh finished him off. Bryan fell against the ropes, dropped to the canvas, and the referee called a halt as the Nottingham fighter sat there.