Palin said in a statement that she and her husband Todd "devote ourselves to God, family and country." She said her decision maintains that order.
Palin was the 2008 GOP nominee for vice president. She often hinted that she might run for president in 2012 but never committed herself.
Her announcement came one day after New Jersey Gov. Chris Christie said he would not run. Republican insiders say the field is set. It includes former Massachusetts Gov. Mitt Romney and Texas Gov. Rick Perry.