Takuma Inoue claimed the WBA bantamweight crown vacated by his superstar older brother Naoya by unanimous decision over Venezuela's Liborio Solis on Saturday.
Takuma's win came three months after Naoya, nicknamed "Monster," relinquished all four of his world bantamweight championship belts to move up in weight to the super bantamweight division.
Takuma, 27, the top-ranked contender, suffered an accidental cut above his left eye during the fifth round, but kept his discipline and composure against his second-ranked 41-year-old opponent at Tokyo's Ariake Arena.
"I'm so relieved that I captured the first of the four belts my brother relinquished," Inoue said. "I'll continue to work hard to achieve my goal of unifying the four.
"I got cut in my face for the first time in a match, but I managed to win and that's a confidence boost."
In another world title match, Kenshiro Teraji successfully defended his WBA and WBC light flyweight titles with a ninth-round technical knockout of Anthony Olascuaga of the United States.
Olascuaga replaced WBO light flyweight champion Jonathan Gonzalez as Teraji's opponent after the Puerto Rican was forced to withdraw after contracting mycoplasma pneumonia.
Earlier, Tenshin Nasukawa, who finished his kickboxing career undefeated last year, made his highly anticipated pro boxing debut with a unanimous decision over Yuki Yonaha in their super bantamweight six-rounder.
Yonaha is Japan's second-ranked domestic bantamweight.