The announcement came just hours before the deadline for lodging applications for the December 23 election -- a vote that analysts say is crucial for the country's future.
Shadary is permanent secretary of Kabila's party, the People's Party for Reconstruction and Democracy (PPRD).
He "will represent our political family in the presidential election," spokesman Lambert Mende told a press conference. "We will all rally behind him."
A country of some 80 million people, the Democratic Republic of Congo has never known a peaceful government transition since it gained independence from Belgium in 1960.
Kabila, 47, has been in power for 17 years, taking over from his father, Laurent-Desire Kabila, who was killed by a bodyguard.
He has been accused by critics of corruption, incompetence and rights abuses. Kabila is barred from running in the polls, according to the two-term limit set in the country's constitution.
But speculation has mounted in recent months that he would try to stand again, perhaps by arguing that he was entitled to do because of a constitutional revision in 2006.
Their choice was "the conclusion of consultations that the president..., the moral authority of the the Common Front for Congo, undertook to find the rare bird," Mende told journalists.
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