Greater Netherlands (Dutch: Groot-Nederland, pronounced [ˌɣroːt ˈneːdərlɑnt]) is an irredentist concept which unites the Netherlands, Flanders, and Brussels. Additionally, a Greater Netherlands state may include the annexation of the French Westhoek, Suriname, formerly Dutch-speaking areas of Germany and France, or even the ethnically Dutch and/or Afrikaans-speaking parts of South Africa. A related proposal is the Pan-Netherlands concept, which includes Wallonia and potentially also Luxembourg.

The Greater Netherlands concept was originally developed by Pieter Geyl, who argued that the "Dutch tribe", encompassing the Flemish and Dutch people, are only separated due to the Eighty Years' War against Spain in the 16th century. While Geyl—an outspoken anti-fascist—argued from a historical and cultural perspective, the fascist Verdinaso and Nazi movements built upon the idea of a Greater Netherlands during the 1930s and 1940s with a focus on ethnic nationalism, a concept still prominent among some on the far-right. Other 21st-century proponents of the Greater Netherlands concept include moderates in Belgium and the Netherlands who seek to elevate the Benelux ideal to a more centralized political union.

Public support for a union of Flanders and the Netherlands is relatively small, especially in Flanders, where Flemish independence is seen as the main alternative to the Belgian state.

Greater Netherlands without Brussels:

Or with Brussels:

The potential country is also known as Dutchland (Dietsland), which incorporates the word Diets – an archaic term for (Middle) Dutch. This label was popular until the Second World War, but its associations with collaboration (especially in Flanders) meant that modern supporters generally avoid using it. The ideology is often labeled as Greater Netherlandism (Groot-Nederlandisme). Dutch Movement (Dietse Beweging) is another term often used for the movement, while in literature it is often called the Greater Netherlands Thought (Grootnederlandse Gedachte).

Greater Netherlandism is often confused with the Orangist movement in Belgium which fought for the reunification of the United Kingdom of the Netherlands after Belgian Independence. While many Orangists are Greater Netherlandists, the Orangists mainly focus on restoring Orange-Nassau's control over the South often for legitimist reasons.

The Prinsenvlag is sometimes used by both Orangist and Greater Netherlandic groups due to its use by supporters of William I of Orange during the Eighty Years' War, who led the revolt of the Low Countries against the Spanish. During this rebellion the Dutch-speaking regions of the Low Countries, encompassing modern day Flanders and the Netherlands, banded together under the Union of Utrecht, the precursor to the modern Dutch state. The flag was also used by the Dutch Republic and United Kingdom of the Netherlands. Today the flag is generally associated with the far-right in the Netherlands.