Ohio v. American Express Co., 585 U.S. ___ (2018), was a United States Supreme Court case regarding the nature of antitrust law in relationship to two-sided markets.

While Visa and MasterCard settled with the United States Department of Justice in 2010, American Express defended its practice by arguing that the anti-steering policies benefited its cardholders, the higher transaction fees helping to maintain member services.

With this decision upheld, the two-market test could be applied to other venues, particularly to the high tech sector where two-side markets are common.

Within the United States, credit card transactions are controlled by four main financial institutions: Visa, MasterCard, American Express, and Discover, making it an oligopoly.

[3] In October 2010, the United States Department of Justice (DOJ), after receiving numerous complaints that the anti-steering language violating antitrust laws and following two years of investigation, filed a civil antitrust lawsuit against Visa, MasterCard, and American Express in United States District Court for the Eastern District of New York.

The DOJ asserted that with US$35 billion in transaction fees each year, the practices of these three institutions prevented merchants from reducing their own costs of business, and requiring them to raise prices to consumers.

In February 2015, following a seven-week trial hearing, the District Court ruled in favor of the DOJ and states, finding that American Express's anti-steering terms violated antitrust laws.

[13] The States' appeal raised questions of the Second Circuit's decision in conflict with past case law, and that Supreme Court guidance was needed on how antitrust measures should be applied a two-sided market, which was becoming a significant portion of the national economy.