Negotiators from the National Liberation Army (ELN) rebel group say they have agreed a temporary ceasefire with the Colombian government, the BBC reports.
It is the first time the two sides have agreed a ceasefire in more than 50 years of conflict.
The announcement comes less than 48 hours before Pope Francis is due to visit Colombia.
Colombian President Juan Manuel Santos said the truce would begin on 1 October and would last until 12 January.
The negotiations in Ecuador’s capital Quito started in February – just months after the Colombian government and the country’s biggest rebel group, the Revolutionary Armed Forces of Colombia (Farc), signed a peace deal. The Farc has since become a political party.
The ceasefire between the smaller ELN and the government was reportedly reached after all-night negotiations.
Negotiations have at times proved frustrating for the government, which has demanded that the group stop kidnapping people.
The ELN finances its operations mostly with the ransom paid to release hostages.
Last weekend, the ELN revealed that it had killed a Russian-Armenian hostage it was holding back in April.