Mandø or Manø (Danish pronunciation: [mænøˀ]) is one of the Danish Wadden Sea islands off the southwest coast of Jutland, Denmark in the Wadden Sea, part of the North Sea. The island covers an area of 7.63 square kilometres (2.95 square miles) and had between 27-31 inhabitants in January 2022. The island is part of Esbjerg Municipality and is situated approximately 12 kilometres (7 miles) southwest of the ancient town of Ribe.

Mandø is barely accessible by road at high tide over an unpaved surface level causeway of about four kilometers in length that connects the island to the mainland. Low tide transit is possible. Extensive mudflats and tidal marshes encircle the island and provide breeding areas to multitudes of birds and other organisms. In the past centuries several large earthen dikes have been constructed around the perimeter of the island, although substantially set back from the shoreline. This artifice has allowed conventional farming in the form of grain growing and sheep grazing. Mandø is technically a hallig, although it is far from the ten German islands commonly described by that term. The name was formerly often spelled Manø.

Conventional motor vehicles can access Mandø Island via a causeway, an unpaved roadway, although this route is compromised in storms at high tide. The nearest village on the mainland which is the gateway to Mandø Island is Vester Vedsted. This simple causeway road is no more than copious gravel laid down on an immense mudflat, with required frequent periodic maintenance of added gravels. Alternatively many visitors reach the island by way of a specially designed tractor-pulled bus with greatly oversized tires. This vehicle is capable of traversing some of the firmer mudflats, but only at the lowest tides. In any case private vehicles or the "Mandø bus" leave the mainland at the point of the Wadden Sea Centre, which offers nature information and boasts a small museum devoted to the natural history of Mandø. Mandø is located midway between the two larger islands Fanø and Rømø which are connected to the mainland by a ferry and a road running across a causeway, respectively.

Mandø is geographically one of the northernmost islands among the North Frisian Islands off the southwestern Danish and northwestern German coast. The Frisian Islands are generally the line of demarcation separating the North Sea from the Wadden Sea, which is a shallow sea of approximately 5,000 square kilometers in size, riddled with small islands and tidal flats. The entire Wadden Sea area was formed by intervening riverine depositional periods between Ice Age glacier movements and the island consists of sedimentary deposits and sand.

The principal ecosystems on this island are: tidal marsh; mudflat; littoral zone; and upland grassland. In fact, there is about as much land area in mudflats as the considerable arable land of the island. Mandø Island is known for its extensive birdlife. Breeding birds consist of terns, sandpipers, many waders and ducks including eiders. Mudflats and marshes afford birds a safe and undisturbed location for feeding, resting and breeding.

There are also a variety of insects, marine organisms and small mammals that find habitat on this island. Many grasses and wildflowers populate the upland portion of the island, in addition to marsh grasses in the intertidal zone. Besides the indigenous wildlife there are sheep which are grazed on the island and along part of the causeway.

The first written record of Mandø dates from 1231. but it is believed people lived there before then, with archaeological remains of Gammel Mandø village, located close to the mainland causeway. In 1558, a violent storm surge raged, and the town of Mandø, which was then located on the northernmost part of Mandø island, was destroyed. The storm surge "split" Mandø into two larger islands at the so-called "Store Lo", and the residents built their new houses on the southernmost of the two islands, "New Mandø", near the current settlement on dunes to the western side. Accretion and dikes have joined them again.

There was a further flood event on 11 October 1634. A 1909 storm surge was only 5 cm from going over the seawall. The years 1909, 1911, 1923, 1928, 1936, 1968, 1976, 1981, and 1999 saw extensive flooding, as marked on a post on the island. Among many storm floods, a dyke known as Bydiget ('village dyke') was constructed in 1887 east of the main settlement, its predecessor having been built largely by women, when men were away at sea. A second one, Havnediget ('harbour dyke'), was constructed in 1937 protecting the northern part of the island, Gammel Mandø ('Old Mandø'). Both dykes were breached in a flood in 1981. Surges are now severe enough to threaten livelihoods and properties.