The Jefferson National Expansion Memorial is a park located in St. Louis, Missouri, near the starting point of the Lewis and Clark Expedition. It was designated as a National Memorial by Executive Order 7523, on December 21, 1935, and is maintained by the National Park Service (NPS).

The park was established to commemorate:

• The Louisiana Purchase, and the subsequent westward movement of American explorers and pioneers;
• The first civil government west of the Mississippi River; and
• The debate over slavery raised by the Dred Scott case.

The memorial consists of a 91-acre (36.8 ha) park along the Mississippi River on the site of the earliest buildings of St. Louis; the Old Courthouse, a former state and federal courthouse that saw the origins of the Dred Scott case; the 45,000 sq ft (4,200 m2) Museum of Westward Expansion; and most notably the Gateway Arch, a steel catenary arch that has become the definitive icon of the city.