The phrase "51st state" refers to territories considered to be potential candidates for addition to the existing 50 states of the United States. For example, Puerto Rico has a possibility to become the 51st state of the U.S. depending on the outcome of a series of referendums on changing the island's political system. Additionally, the phrase can also refer negatively to independent states/countries perceived to be under excessive American influence or control.

Republican Jeff Stone has asked 13 conservative counties in Southern California to break away from the rest of California to create the 51st state of the United States.