The unnamed envoy will visit the North's capital city on September 5, Yonhap news agency said, citing a presidential spokesman.
Seoul proposed the envoy's visit Friday morning and Pyongyang accepted it a few hours later, Kim Eui-kyeom said, adding they had not yet who the envoy will be.
"The envoy will have broad discussions over a detailed schedule for the inter-Korea summit, development of bilateral ties...and nuclear disarmament of the Korean peninsula," Kim was quoted as saying.
Moon and Kim have met face-to-face twice now, the first during a historic summit at the border truce village of Panmunjom in April.
It was the first time a North Korean leader had ever crossed into the South.
They met a second time in the truce village as they scrambled to salvage a summit between Kim and US president Donald Trump in Singapore which eventually went ahead.
They have since agreed to hold a third summit in Pyongyang at an unspecified date in September.