Hyon Yong Chol, who headed the isolated nuclear-capable country's military, was charged with treason, including disobeying Kim and falling asleep during an event at which North Korea's young leader was present, according to South Korean lawmakers briefed in a closed-door meeting with the spy agency on Wednesday.
His execution was watched by hundreds of people, according to NIS intelligence shared with lawmakers.
It was not clear how the NIS obtained the information and it is not possible to independently verify such reports from within secretive North Korea.
"The NIS official said it had been confirmed by multiple sources. It is still just intelligence, but he said they were confident," Shin Kyoung-min, a lawmaker and member of the opposition New Politics Alliance for Democracy, who attended the briefing, told Reuters.