The Sports Illustrated model was reported to have been spotted kissing the rapper at Club LIV on Miami Beach earlier this week.
An alleged eyewitness was quoted as telling the New York Daily News: "They were kissing, they weren't hiding it."
However, 20-year-old Upton confirmed that the reports were false on her Twitter page.
Diddy - also known as Puff Daddy, P Diddy and his real name Sean Combs - also denied being close to the model.
Attention all Media. I don't even know Kate Upton personally! I'm not dating her ! What's being reported is not true. END of story!— iamdiddy (@iamdiddy) April 10, 2013
Upton ended her relationship with Detroit Tigers pitcher Justin Verlander in February.
Last month, the model agreed to a prom date with a high school student in the US.
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