The sale took place Sunday during a natural history-themed auction at Bonhams in Los Angeles.
Bonhams says the tooth measuring more than five inches is among the biggest ever discovered and was found in Montana's Garfield County earlier this year.
Spokeswoman Jannelle Grigsby says it sold for $56,250 to a private collector whose name was not released.
The T. rex terrorized western North America some 67 million years ago.
She says a fossil palm flower from Wyoming sold for $50,000.