Marstal municipality, is a former municipality (Danish, kommune) in Funen County on the island of Ærø in southern Denmark. Since 1 January 2006 it has been part of Ærø municipality. Its territory included the islands of Birkholm and Halmø, and covered an area of 17 km². In 2005, it had a total population of 3,208. Its last mayor was Karsten Landro, a member of the Conservative People's Party (Det Konservative Folkeparti) political party.