What document allowed the US to purchase the land west of the Mississippi River from France?

The document that allowed the United States to purchase land west of the Mississippi River from France is the Louisiana Purchase Treaty, signed on April 30, 1803. This agreement, negotiated by President Thomas Jefferson, resulted in the U.S. acquiring approximately 828,000 square miles of territory, effectively doubling the size of the nation. The purchase was finalized for $15 million, making it one of the largest land deals in history.