Jamaal Franklin had 23 points and Chase Tapley 19 for the Aztecs, regular-season champs of the Mountain West.
Georgetown had lost early in the tournament the last two years, but made sure that didn't happen this time.
With a ruthless defense that bottled up Belmont (27-8), the Hoyas forged a nine-point halftime lead and led by as many as 19 points near the finish.
"It was so good to get that first game," said Hoyas center Henry Sims, who had 15 points. "But we didn't come here just to win one game."
Georgetown's previous two seasons ended with embarrassing opening-round losses to Virginia Commonwealth and Ohio, scrappy mid-majors who made the big, bad Hoyas look silly. This time around, Georgetown played like the Georgetown of old, using inside muscle and a swarming defense that choked the life out of the Bruins.
"They're well-taught, and they've got great athletes," Belmont coach Rick Byrd said. "If (Big East champion) Syracuse is better than that, I'm glad we didn't play Syracuse."
AP Sports Writer Tom Withers contributed to this story.
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