The declaration came the day before a rare meeting of the party's top delegates.
Kim Jong Un had been elected as a delegate to the meeting, a necessary first step to succession of power, South Korean newspaper Chosun Ilbo reported Monday.
In announcing the party congress, the Workers Party said it was to "elect new members of the top party apparatus."
The committee declared Kim Jong Un "the only successor to leader Kim Jong Il," the newspaper reported.
If he is to succeed his father, Kim Jong Un must first be elected to the Central Committee. His father was established as the country's leader after he became a member of the Central Committee in 1972.
Party leaders gathered in the North Korean capital of Pyongyang Monday --- the day before the announced meeting.
Analysts of North Korea's politics had said the ailing Kim would likely appoint his son as his successor, The New York Times reported.
"This is a party meeting, and these are very rare in North Korean history," said John Delury, professor of international relations at Yonsei University in Seoul. He was in Pyongyang last week, and met with North Korean officials.
Tuesday's session will be the first of its kind in 30 years, the Yonhap News Agency said.
Kim Jong Il, 68, reportedly had a stroke in 2008 and his health may be rapidly declining, necessitating his naming of a successor quickly, observers speculated.