"I am done hiding and will say this unequivocally: Jon is a violent and abusive person," the 45-year-old reality star told People on Wednesday.
Last week, Kate said Pennsylvania Child & Youth Services had alerted her that they were looking into the claims, but later released a statement saying, "No charges or citations were filed as a result of that incident. We are not investigating at this point."
Kate, however, said she believes there is more work to be done with the investigation.
"Until I receive the letter from CYS about whether they've deemed the allegations founded or not, it's ongoing," she told People. "My contact at CYS has not told me otherwise. If this doesn't qualify as assault, I don't know what does."
On September 3, Collin accused his father of abuse, writing in a since-deleted post, "My dad is a liar. Yesterday he beat me up and thought nothing of it, he punched me in the face and gave me a swollen nose and I started bleeding. He then continued to kick me in the ribs after I was on the floor. He is a liar."
No charges have been brought against Jon and he has denied all allegations.
"Jon has never abused Collin," a rep told E! News. "Collin has endured severe trauma. Jon is a loving father who has worked hard to ensure that his son gets the support and help he needs."
Kate and Jon divorced in 2009 after ten years of marriage. They shot to fame in 2007 with their reality show "Jon & Kate Plus 8".
19-year-old twins Cara and Madelyn live with their mother, as do four of the 16-year-old sextuplets Alexis, Aaden, Joel and Leah; while the remaining two siblings — Hannah and Collin — live with their dad.