The U.S. president known for the Louisiana Purchase is Thomas Jefferson. He served as the third president from 1801 to 1809 and was pivotal in expanding the United States' territory. The Louisiana Purchase, finalized in 1803, involved buying approximately 828,000 square miles of land from France for $15 million. This acquisition doubled the size of the U.S., providing vast lands for westward expansion and significantly impacting the nation's economic and political landscape. Jefferson believed that this expansion aligned with the American ideal of democracy, though it also led to complex issues regarding the treatment of Native Americans and slavery.