A natural harbor is a landform where a body of water forms a harbor. Natural harbors have long been of great strategic and economic importance. Many of the great cities of the world are located on a natural harbor. There is no dispute that Sydney Harbour is the world's largest natural harbor, but the identity of the second largest is a matter of controversy: several towns and cities across the world claim to possess the second largest. These places include:

• Rio de Janeiro, Brazil
• Halifax, Nova Scotia, Canada
• Falmouth, Cornwall, United Kingdom
• Poole, Dorset, United Kingdom
• Cork, Republic of Ireland