Congress officially began using the name “United States” instead of “United Colonies” on September 9, 1776.
The change occurred during the Revolutionary War, after Congress had adopted the Declaration of Independence. The new wording appeared in congressional proceedings and reflected the political transformation from colonies within the British Empire to states claiming sovereignty.
The Declaration itself used the phrase “these United Colonies” in its opening and referred to the “united States of America” near its conclusion. Congress's September action made “United States” the formal short name used in its official records.
This naming decision is different from the adoption of the Declaration on July 4 and from the Articles of Confederation, which were completed in 1777 and ratified in 1781. It also predates the Constitution's opening words, “We the People,” written in 1787. The change captured a new national identity while the war and political settlement were still unfinished.