Dual loyalty is when citizens of one state whose cultural or religious affiliation with another country is strong have a loyalty to the other country which equals or exceeds their loyalty to their home country.

An example of the dangers of dual loyalty is aptly demonstrated by the history of the Second French Empire. Napoleon III was in his early life a member of the Italian anti-Austrian resistance movement, the Carbonari. This led to an unhealthy anti-Austrian bias in French foreign policy which resulted in France's failure to prevent the rise of Prussia, leading ultimately to France's disastrous defeat in the Franco-Prussian War.

Accusations of dual loyalty are often leveled against those of minority religious views who feel a loyalty to their faith as well as their country. For example, the charge of dual loyalty is often applied to Jews outside of Israel, especially Jewish neoconservatives who promote hawkish foreign policy in the Middle East. Such accusations of dual loyalty are generally considered to be a form of anti-Semitism, especially when no additional justification for the charge is given.

Similarly, Catholics have been accused of dual loyalty due to their affiliation with the Pope; in particular, this was widely urged in the United States as a reason not to vote for John F. Kennedy for president in 1960.