31-year-old Gambhir will replace New Zealand batsman Hamish Rutherfod as the county's overseas recruit. Rutherford will be heading home to prepare for New Zealand's upcoming international commitments.
"Hi guys, off to play county cricket tomorrow, will be playing for Essex Eagles," Gambhir wrote on his twitter page.
Gambhir is expected to join the team ahead of the LV County Championship match against Northamptonshire in Colchester, which begins on August 20.
"He will no doubt bring his experience to the squad during an important stage of the season for us in the LV County Championship and Yorkshire Bank 40 competition."
For nearly five years, Gambhir was India's first choice opener across all three formats before prolonged bad patch forced the selectors to drop him from the national squad.
Gambhir has played 54 Tests for India and scored 4021 runs at an average of 44.18. Gambhir is the first Indian player to sign for Essex this season after Harbhajan Singh played for the same county last season.