Sharma, a 1976 batch Kerala cadre IPS officer, retired as director of the IB on December 31, 2016.
He was appointed as the interlocutor for talks with Assam-based insurgent groups last week.
Sharma took charge yesterday, official sources said.
Sharma, who has been appointed for a tenure of one year, will hold peace negotiations with pro-talks factions of the ULFA, National Democratic Front of Bodoland (NDFB) and a few other small groups based in Assam.