Foster v. Neilson, 27 U.S. 253 (1829) was a decision by the United States Supreme Court that held certain treaties ratified by the United States, even if otherwise valid and in force, cannot be given effect domestically without a specific act of Congress.

[1][2] The ruling articulated a more restrictive interpretation of the Supremacy Clause of the U.S. Constitution, which automatically grants treaties the force of domestic federal law.

[3] The Foster decision was the first to formulate the concept of "self-execution", which distinguishes between treaties that are "self-executing" (meaning domestic courts can enforce them directly upon their ratification) and those that are "non-self-executing" (which are not directly enforceable in U.S. courts unless Congress passes specific implementing legislation).

[4] The Court's opinion, authored by Chief Justice John Marshall, recognized that the U.S. Constitution, through the Supremacy Clause, "declares a treaty to be the law of the land" and "consequently to be regarded in courts of justice as equivalent to an act of the legislature."

"[5] Using this test, the Foster Court held that the treaty provision at issue—which stated that certain land grants from the King of Spain "shall be ratified and confirmed"—was non-self-executing because it suggested that Congress would ratify the land grants through a future legislative act.