In international law and business, patent trolling or patent hoarding is a categorical or pejorative term applied to a person or company that attempts to enforce patent rights against accused infringers far beyond the patent's actual value or contribution to the prior art, often through hardball legal tactics (frivolous litigation, vexatious litigation, strategic lawsuits against public participation (SLAPP), chilling effects, and the like). Patent trolls often do not manufacture products or supply services based upon the patents in question. However, some entities which do not practice their asserted patent may not be considered "patent trolls" when they license their patented technologies on reasonable terms in advance.

Other related concepts include patent holding company (PHC), patent assertion entity (PAE), and non-practicing entity (NPE), which may or may not be considered a "patent troll" depending on the position they are taking and the perception of that position by the public. While in most cases the entities termed "trolls" are operating within the bounds of the legal system, their aggressive tactics achieve outcomes contrary to the origins of the patent system, as a legislated social contract to foster and protect innovation; the rapid rise of the modern information economy has put the global intellectual property system under more strain.

Patent trolling has been less of a problem in Europe than in the United States because Europe has a loser pays costs regime. In contrast, the U.S. generally employs the American rule, under which each party is responsible for paying its own attorney's fees. However, after the U.S. Supreme Court's decision in Octane Fitness, LLC v. ICON Health & Fitness, Inc. on April 29, 2014, it is now easier for courts to award costs for frivolous patent lawsuits.