The Duchy of Grand Fenwick is a tiny fictional country created by Leonard Wibberley in a series of comic novels, beginning with The Mouse That Roared (1955).

The Duchy is described as bordering Switzerland and France in the Alps, and it proudly retains a pre-industrial economy, dependent almost entirely on making Pinot Grand Fenwick wine. The Duchy takes his name from its founder, the English knight Sir Roger Fenwick, who settled there with his followers in 1370. The nation's principal language is English. Its ruler is the Duchess Gloriana XII.

Wibberly places the tiny country (3 miles by 5 miles) in a series of absurd situations, where it goes up against superpowers and invariably wins. It defeats the United States in a war (The Mouse That Roared) by threatening the use of the Q-bomb (the secret ingredient for which occurs in a particular batch of Grand Cru Pinot Grand Fenwick), beats the U.S. and Soviet Union in a space race (The Mouse on the Moon (1962)) and disrupts the world's finances (The Mouse on Wall Street (1969)). Wibberley generally goes beyond the merely comic, using the situations to make commentary about modern politics and world situations.

Grand Fenwick also appeared in film in 1959, when The Mouse That Roared was made into a movie starring Peter Sellers in three different roles (including the Duchess). In 1963, The Mouse on the Moon was made into a movie, but without Sellers.

Fictional countries