President Juan Manuel Santos says the armistice between the government and the Latin American country's last active group will come into effect on October 1.
Colombia's government and the ELN, the country's last active guerrilla group, have agreed to a ceasefire after months of talks, the rebels announced Monday.
The announcement in the Ecuadorian capital Quito, where the talks are being held, comes on the eve of a visit by Pope Francis to Colombia.
"Yes, it was possible," the National Liberation Army (ELN) delegation said in a tweet announcing the deal.
The latest round of talks between the government of President Juan Manuel Santos and the ELN have been ongoing in neighbouring Ecuador since February.
Colombia's biggest rebel force, the FARC, disarmed last month under a peace deal with the government to end more than half a century of armed conflict.
Santos now wants a deal with the ELN to seal a "complete peace."
Decades of war
The FARC and ELN formed in 1964 to fight for land rights and protection of poor rural communities.
The conflict drew in leftist guerrillas, right-wing paramilitary groups and state forces.
It left 260,000 people confirmed dead, more than 60,000 missing and seven million displaced.
Officials say remnants of right-wing paramilitary groups are still fighting the ELN for control of the drug trade.
Both sides discussed the possibility of agreeing to a temporary ceasefire before the arrival of the pope, who begins a four-day visit to Colombia on Wednesday.
"We have said that the visit of Pope Francisco should be an extra motivation to accelerate the search for agreements, which have as the main target communities that suffer the unfortunate consequences of the conflict," the group said on another of its accounts on Twitter, using the Spanish version of the pope's name.