Roh and Kim weren't scheduled to meet until Wednesday, but Kim appeared at a ceremony where Roh and first lady Kwon Yang-suk were being welcomed by Kim Yong Nam, North Korea's titular head of state, the Yonhap news agency reported.
The report said the two men shook hands, and each said, "Glad to meet you," although no other exchanges were observed during a 12-minute ceremony.
Hundreds of thousands of North Koreans lined the streets of Pyongyang as an open motorcade carried Roh, his wife and North Korean officials to the People's Palace of Culture, the report said.
Two rounds of summit talks are scheduled for Wednesday between Roh and Kim. Agenda items include discussion of the end to the North's nuclear program, turning a decades-old cease-fire into a peace treaty and eventual reunification, the report said.