The United States Congress adopted the Articles of Confederation as its first constitution in 1777.
The Second Continental Congress approved the Articles on November 15, 1777, after more than a year of debate. The document created a “firm league of friendship” among the states and provided a framework for conducting the Revolutionary War and foreign affairs. It did not create the strong central government later established by the Constitution.
The Articles did not take effect immediately upon congressional approval. Maryland's objections to western land claims delayed final ratification, and the last state ratified on March 1, 1781. The Confederation Congress then operated under the Articles until the Constitution replaced them in 1789.
A common mix-up is treating 1781 as the adoption year. That was the year the Articles became operative after final ratification. Another is calling the Declaration of Independence the first constitution; the Declaration announced separation from Britain, while the Articles organized the new union.