The National Intelligence Service (NIS) dismissed reports that claimed Kim Yo-jong ousted Kim Jong-un after a coup.
A separate government official also said such reports are not true.
Citing a report from US-based tabloid "Globe", some news outlets here said Kim Yo-jong staged a coup between May 6 and June 5, and removed Kim Jong-un from the post.
The report claimed Kim Jong-un at recent events is actually an impostor, pointing out that his appearance is different than the past, with substantial weight loss.
Speculations about Kim Jong-un's well-being have been brought up by media regularly since last year.
In July, the NIS dismissed rumours over Kim Jong-un's health problems as "groundless".
The North Korean leader earlier this month attended a defence development exhibition in Pyongyang and called for boosting military capabilities.
He was also seen smoking cigarettes with officials at the event, according to photos released by the North.