The club announced on their website that Martin, who has not played for the first team since the club was promoted in 2012, has left to play regular football elsewhere.
The 24-year-old picked up a serious injury in the Development Squad's friendly against Eastleigh last summer, and only returned to action this month.
The Isle of Wight-born defender started his career with non-league side Eastleigh and signed for Southampton in November 2009. He made his debut in May 2010 and the following season played 10 times as Southampton won promotion to the Championship.
Martin moved on loan to Crystal Palace in July 2012 but only made six appearances before heading to Coventry, where he played 12 times.