The gender and name of the child are unknown, Yonhap News Agency reports.
According to information from the NIS, Ri gave birth to her first child in 2010 and second one in 2013.
Kim is the third generation of his dynasty to rule North Korea, but little has been revealed about the country's first family.
"The NIS told us (Monday) about Ri Sol Ju giving birth to the third child because this implies she remains an influential figure in North Korea", Yi said.
It was only through former National Basketball Association player Dennis Rodman that Kim and Ri's second child's details emerged after the American visited the reclusive country in 2013. Rodman later went on to describe Kim as "a good dad".
News of the new arrival emerged as North Korea fired a ballistic missile over northern Japan.
Ri disappeared late past year, leading some to assume that she was pregnant.
It wasn't until the following year that South Korean intelligence confirmed they had been married since 2009, according to lawmakers briefed on the matter.
She visited South Korea in 2005 as a cheerleader for her country's squad in the Asian Athletics Championships.
This photo, released by the North's Korean Central Television on March 3, 2017, shows North Korean leader Kim Jong-un and his wife Ri Sol-ju meeting students of the Red Flag Mangyongdae Revolutionary School in Pyongyang.
Initially, North Korea watchers expected Kim Jong Il's eldest son, Kim Jong Nam, to succeed him, but he was sidelined after his father left his mother for Ko in the 1970s. (RELATED: Kim Jong Un's Wife Has Disappeared And Rumors Are Flying) Ri is somewhat of a mystery, much like most of the Kim family.