He will replace New Zealand player Hamish Rutherford as Essex's overseas player.
"Hi guys, off to play county cricket tomorrow, will be playing for Essex Eagles," Gambhir tweeted on Thursday.
Gambhir has scored 4021 runs in his 54 Test appearances for India.
He has played 147 One Day Internationals (ODIs) for India and scored 5238 runs.
Gambhir has represented India in 37 T20 matches in his career so far.
He was not selected for the Champions Trophy and the tri-series in West Indies recently.
--IBNS (Posted on 15-08-2013)