Timochenko was previously the supreme commander of the Revolutionary Armed Forces of Colombia - acronym FARC in Spanish -- a Marxist guerrilla group that for half a century battled the government and rightwing paramilitaries.
The conflict, marked by kidnappings and disappearances, left some 260,000 people dead, 60,000 unaccounted for and seven million displaced.
The FARC rebels agreed in 2016 to a landmark peace deal with the government. It disarmed and last month transformed itself into a political party, keeping the same initials but changing its official name to the Common Alternative Revolutionary Force.
Just ahead of the political FARC's founding congress in August, Timochenko -- who suffered a temporary arterial brain blockage in July, impeding his speech -- had ruled out the party fielding a presidential candidate next year.
But the party in early September named Timochenko its leader, and with Wednesday's announcement showed it intended to have him try to succeed current President Juan Manuel Santos.
The party also put forward four prominent members as candidates for congressional elections in March 2018. They included Ivan Marquez -- a former commander and party spokesman who declared Timochenko's candidacy -- and three former negotiators who saw through the peace deal.
The FARC, however, has a negative image for the majority of Colombians, according to surveys, making its election bids uncertain.
Under the peace deal, former rebels and government soldiers guilty of crimes during the conflict can receive non-prison punishment if they confess, make reparations to victims, and renounce violence.
It is unclear if FARC candidates can forge political careers before being judged by an extraordinary tribunal, a Special Jurisdiction for Peace.
Under the peace deal's terms, the FARC is guaranteed 10 seats in Colombia's Congress for two terms, though the party must take part in elections.
The UN has also heard concerns about progress under the peace deal.
In October, UN assistant secretary-general for human rights Andrew Gilmour warned that reintegration of former FARC fighters "is not going so well."
Many are finding it difficult or impossible to return to civilian life, raising the risk they might turn to crime rings, illegal mining or drugs, he said.
"If you don't reintegrate the fighters then there is a strong chance that they will go back to something worse, even if they've given up their weapons," Gilmour told reporters at UN headquarters on October 20.