Wicketkeeper James Foster will captain Essex for the remainder of the county's Friends Provident t20 campaign.
The England international replaces Mark Pettini in the role.
Pettini will continue his duties as club captain for Essex, who are sixth in the South group of the Twenty20 competition, having won one of their three matches to date.
Essex head coach Paul Grayson said: "This change will come in with immediate effect.
"James (Foster) will captain the side from tomorrow (at Middlesex) and for the rest of the tournament, including the Twenty20 game against Pakistan."