South Korea's National Intelligence Service reportedly told the intelligence committee in parliament of the birth on Monday. The baby's gender and name were not provided.
Kim and his wife, Ri Sol Ju, have two other children who were believed to have been born in 2010 and 2013.
Former US basketball star Dennis Rodman, who traveled to North Korea in 2013, has said Kim's second child is a girl named Ju Ae, but the first child's gender remains unknown, Yonhap said.
Ri was rumored to be pregnant after being out of public view for the past several months, the report said.
The news about the third child's birth came the same day that North Korea fired a ballistic missile over the Japanese island of Hokkaido before landing in the Pacific Ocean.