The deal was signed between senior representatives of the anti-balaka rebels and the ex-Seleka movement of ex-president Michel Djotodia.
CAR is struggling to recover from the coup that ousted president Francois Bozize and pushed the poor and unstable country into violence pitting the country's Christian and Muslim populations against one another.
"The parties have adopted a ceasefire, a cessation of hostilities, and a DDRR (Disarmament, Demobilisation, Rehabilitation and Reintegration) agreement," mediator Kenneth Marende, a former speaker of Kenya's parliament, told AFP.
Senior representatives of the anti-balaka rebels and the ex-Seleka movement have been meeting in Kenya since early December, in talks parallel to meetings in the Democratic Republic of Congo, where the two sides have signed a previous tentative ceasefire.
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Marende declined to give the exact names of those who signed the deal in Nairobi.
"Rest assured, they are the genuine leadership," Marende said. "Such things must remain confidential at this stage of mediation."
The deal was struck on Friday in Kenya, with diplomats saying it took place in an upmarket hotel in Nairobi. No timeframe has been given for the next stage of talks.
Violence between rival factions has plunged the deeply impoverished country into an unprecedented political and security crisis.
The mainly Muslim Seleka seized power in March 2013 and put Djotodia -- the country's first Muslim president -- in power.
Djotodia stepped down in January 2014 under strong international pressure for his failure to rein in rogue ex-rebels, who relentlessly murdered, raped and stole from civilians.
In response, largely Christian communities formed "anti-balaka" -- or anti-machete -- vigilante forces who hunted down Muslims in revenge attacks.
Interim President Catherine Samba-Panza took over power, but the government is not part of this peace process.