The pair have been dating publicly since September 2012, when they were seen together at a Swansea City match during which Coleman, 43, was scouting for international selection.
Jackson, 34, posted on her Twitter page: "Been made a very happy girl & taken off the shelf! Ha." The post was accompanied by a picture of Jackson sporting an engagement ring.
Coleman has also managed Real Sociedad, Coventry City, Larissa and Fulham, where he spent five years of a playing career that was ended prematurely by injury.