The Dancing with the Stars contestant told The Billy Bush Show on Wednesday that allegations of a relationship with Kourtney Kardashian's boyfriend Disick back in 2009 are "100% not true".
The report had claimed that Cavallari and Disick got together while he was on a break with Kardashian and before she became pregnant with their son Mason.
Former Hills star Cavallari went on to tell Bush that the Kardashian family should have spoken out against the report sooner and suggested that the timing of the story was "convenient" for them.
"I just think, what convenient timing for them with their show premiering next week and with everything that's going on for Kim [Kardashian], I feel like they're trying to take the heat off her," she told Bush.
As for what her boyfriend Jay Cutler thought of the claims, she replied: "He didn't believe it for one second."
Cavallari became engaged to Cutler in April although they were reported to have briefly separated a few months later, only to reunite during her time on Dancing with the Stars this season.
Kardashian and Disick haven't commented on the report.