The reality TV star, who was sued last week by Michael Girgenti, a male model who claimed to be Mason's father, spoke out after lab results showed that her long-term boyfriend was in fact Mason's dad.
"After three-and-a-half years of rumors and lies being spread by an individual I met briefly at a photo shoot, I am setting the record straight that Scott is Mason's father," Kardashian told UsWeekly.
"While it saddens me to have to address these ridiculous lies -- especially when the truth was never in doubt -- this story must be put to rest."
E! News published a lab test result document that states that Disick is, with 99.99 percent probability, Mason's father.
Kardashian's lawyer Todd Wilson told TMZ that Girgenti's claims were "false and defamatory."
"We have not seen the reported court filing, but we are informed that this individual has been seeking to get paid to publish his ridiculous pleading," Wilson said. "His claims are absurd and this is simply another shameful attempt to gain publicity and profit from fictional stories about my client."
In his suit, Girgenti requested joint custody of Mason in addition to DNA tests for Kardashian, Disick, Mason and himself. The model claimed he fathered Mason after he and Kardashian had an unprotected sexual encounter in 2009.