Kim voiced his opinion in an interview with the Daily Mail Australia while discussing the dangers of prescription drug misuse. Ledger died at age 28 in 2008 from an accidental drug overdose.
"It was totally his fault. It was no one else's. He reached for them. He put them in his system. You can't blame anyone else in that situation," Kim said. "That's hard to accept because I loved him so much and was so proud of him."
Ledger, who died of cardiac arrest, was later revealed to have several drugs, including oxycodone, hydrocodone and diazepam, in his system. Kim stood by his blunt assessment of his son's death on Thursday's episode of The Morning Show.
"What I said is what I feel," he said. "We are responsible, in most cases, for what we put in our mouths ... In Heath's case, his was just a tragic mix of a prescribed drug with sleeping tablets that just put him to sleep permanently."
Kim told news.com.au his son's death was "a one-off" accident and not the result of addiction, but admitted the actor struggled with "self-induced" pressure "to perform." Ledger's sister, Kate Ledger, reportedly warned him about mixing medications the night before his death.
"The last conversation Kate had with him was this discussion about his medications," Kim said. "He was warned by his sister the night before: 'You shouldn't mix what you're taking for pneumonia with your Ambien.'"
Ledger left behind daughter Matilda Ledger, now 10, who lives with her mother, actress Michelle Williams. The late actor was known for such films as Brokeback Mountain and The Dark Knight.