The 33-year-old reality TV star was asked by male model Michael Girgenti to allow Mason, three, to undergo the test, after he claimed they slept together while she and Scott, 30, were on a break in March 2009 but lab results have proved his allegations are false.
Kourtney, who has always denied Michael's allegations, told E! News: ''After three-and-a-half years of 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. 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.''
Scott and Kourtney also have a 13-month-old daughter Penelope.
This is not the first time the Kardashian family has been confronted by paternity allegations.
Kourtney's younger sister Khloe, 29, has had to regularly deny claims that their late father Robert Kardashian was not her biological father.
Their half-sister Kylie Jenner, 15, also recently hit back at reports that her father Bruce Jenner may not be her real father.
Recently, she shared her disgust in relation to the allegations with her fans on Twitter, writing: ''Yes, my father is my father and no my parents aren't getting a divorce. You guys can't believe any of these magazines. It's all bulls**t.
''And even though I don't let these stories get to my head. Imagine you at 15 & someone telling you your father isn't your father.
''EWWWW people disgust me. You're all pathetic.
''Wait. But you're ACTUALLY pathetic. Going after a 15-year-old and telling her father isn't her father. Like WHAT? Are you insane?
''This makes me so mad. And I'm not taking the time to write all this because this hurts me. I'm writing this for the other teens my age.''
Spencer Morrill believed he was in a relationship with the singer and that she was hiding messages for him in her songs.
Was the singer unknowingly taking counterfeit pain medication?
Roberts had left the band in 2012, due to problems with blood circulation and other health issues.
The popstar turned actress is back in the studio.