James Bradley Thayer (January 15, 1831 – February 14, 1902) was an American legal theorist and educator known for articulating the concept of rational basis review.

Born at Haverhill, Massachusetts, he graduated from Harvard College in 1852, where he established the overcoat fund for needy undergraduates.

[2] The concept of rational basis review can be traced to his influential 1893 article, "The Origin and Scope of American Constitutional Law."

Thayer argued that statutes should be invalidated only if their unconstitutionality is "so clear that it is not open to rational question.

"[5] Justice Oliver Wendell Holmes Jr., a student of Thayer, articulated a version of what would become rational basis review in his canonical dissent in Lochner v. New York and argued that "the word 'liberty' in the Fourteenth Amendment is perverted when it is held to prevent the natural outcome of a dominant opinion, unless it can be said that a rational and fair man necessarily would admit that the statute proposed would infringe fundamental principles as they have been understood by the traditions of our people and our law."