Herman Webster Mudgett (May 16, 1861 – May 7, 1896), better known as Dr. Henry Howard Holmes or, more commonly, as H. H. Holmes, was an American serial killer. While he confessed to 27 murders, only nine could be plausibly confirmed and several of the people he claimed to have murdered were still alive. He is said to have killed as many as 200, though this figure is only traceable to 1940s pulp magazines. Many victims were said to have been killed in a mixed-use building which he owned, located about 3 miles (5 km) west of the 1893 World's Fair: Columbian Exposition, supposedly called the World's Fair Hotel (informally called "The Murder Castle"), though evidence suggests the hotel portion was never truly open for business.
Besides being a serial killer, Holmes was also a con artist and a bigamist, the subject of more than 50 lawsuits in Chicago alone. Many now-common stories of his crimes sprang from fictional accounts that later authors assumed to be factual. In a 2017 biography, Adam Selzer wrote that Holmes' story is "effectively a new American tall tale – and, like all the best tall tales, it sprang from a kernel of truth".
H. H. Holmes was executed on May 7, 1896, nine days before his 35th birthday, for the murder of his friend and accomplice Benjamin Pitezel. During his trial for the murder of Pitezel, Holmes confessed to many other killings.