The announcement deprives Republicans of one of their leading gubernatorial candidates in the nation and opens up the race to a slew of Democrats who are already running.
Rell informed reporters of her decision during a press conference Monday.
Already running on the Democratic side are 2006 Senate nominee Ned Lamont, Connecticut Secretary of State Susan Bysiewicz, Stamford Mayor Dan Malloy, and former state House Speaker Jim Amann.
Potential Republican candidates include Lt. Gov Michael Fedele, state House Minority Leader Larry Cafero (who has an exploratory committee), state Senate Minority Leader John McKinney, who turned down a run against Rep. Jim Himes (R-Conn.) earlier this year.
Rell replaced Gov. John Rowland (R) in 2004 when he resigned amid a corruption investigation, and she was elected to a full term in 2006.
She has remained one of the most popular governors in the country, even as she has seen her popularity wane a bit in recent months. A recent Quinnipiac poll had her job approval at 59 percent.