A personal union is the combination by which two or more different states have the same monarch while their boundaries, their laws and their interests remain distinct. It should not be confused with a federation which is internationally considered a single state. Nor is it to be confused with dynastic union, where the union can be under a dynasty.

Personal unions can arise for very different reasons, ranging from coincidence (a princess who is already married to a king becomes queen regnant, and their child inherits the crown of both countries) to virtual annexation (where a personal union sometimes was seen as a means of preventing uprisings). They can also be codified (i.e., the constitutions of the states clearly express that they shall share the same person as head of state) or non-codified, in which case they can easily be broken (e.g., by the death of the monarch when the two states have different succession laws).

Because presidents of republics are ordinarily chosen from within the citizens of the state in question, personal unions are almost entirely a phenomenon of monarchies, the unique exception in modern times being the Principality of Andorra in which one of the two co-princes is the President of France, while the other is a Roman Catholic bishop. Sometimes the term dual monarchy is used to signify a personal union between two monarchies.