There is no official information available about the marital history of the 64-year-old leader of the highly secretive government, but Kim is widely believed to have been married three times. His last wife, Ko Yong Hi, reportedly died of cancer in 2004.
Since then, Kim has been living with Kim Ok, who had served as his personal secretary since the 1980s, Yonhap reported, citing South Korean government officials it didn't further identify.
Kim Ok, 42, "virtually acts as North Korea's first lady" and frequently accompanies the North Korean leader on his visits to military bases and in meetings with visiting foreign dignitaries, Yonhap said.
She also traveled with Kim Jong Il on a secretive trip to China in January, when she was received by Chinese officials as Kim's wife, the report said. Kim Ok also met with Chinese President Hu Jintao, it said.
It is not known whether Kim Ok and the North Korean leader have any children, Yonhap said.
Kim Jong Il is known to have three sons — one from his second wife and two from his third. North Korean experts say Kim's 25-year-old son Kim Jong Chul is most likely to become the North's next leader.
His eldest son, Kim Jong Nam, is said to have fallen out of favor after embarrassing his father in 2001 when he was caught trying to enter Japan on a fake passport, saying he wanted to visit Tokyo Disneyland.