The title bout was the main event of ShoXC: Elite Challenger Series at the Chumash Casino Resort in Santa Ynez, California.
The judges scored the bout 49-46, 49-46 and 48-47 for Weis.
"This was my dream, to win the 140-pound world title," Weis said. "This is all very exciting."
The ground game of Weis, a Brazilian Black Belt and grappling champion, was the deciding factor against King of the Cage 135-pound champ Cullum.
"No one wants to lose, but he was a strong guy and tough for me to keep off," Cullum said. "I wanted to be the aggressor, but I wound up on the ground too much. I thought the referee could have stood us up a couple of times, but it didn't happen."
Reis improved to 6-0 while Cullum, who entered the fight on ten consecutive wins, fell to 11-2.
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