Henry Louis Mencken (September 12, 1880 – January 29, 1956) was an American journalist, essayist, satirist, cultural critic, and scholar of American English. He commented widely on the social scene, literature, music, prominent politicians, and contemporary movements. His satirical reporting on the Scopes Trial, which he dubbed the "Monkey Trial", also earned him attention. The term Menckenian has entered multiple dictionaries to describe anything of or pertaining to Mencken, including his combative rhetorical and prose styles.

As a scholar, Mencken is known for The American Language, a multi-volume study of how the English language is spoken in the United States. As an admirer of the German philosopher Friedrich Nietzsche, he was an outspoken opponent of organized religion, theism, censorship, populism, and representative democracy, the last of which he viewed as a system in which inferior men dominated their superiors. Mencken was a supporter of scientific progress and was critical of osteopathy and chiropractic. He was also an open critic of economics.

Mencken opposed the American entry into World War I and World War II. Some of the opinions in his private diary entries have been described by some researchers as racist and antisemitic, although this characterization has been disputed. Larry S. Gibson argued that Mencken's views on race changed significantly between his early and later private writings which started when he was 50, and that it was more accurate to describe Mencken as elitist rather than racist. He seemed to show a genuine enthusiasm for militarism but never in its American form. "War is a good thing," he wrote, "because it is honest; it admits the central fact of human nature... A nation too long at peace becomes a sort of gigantic old maid."

His longtime home in the Union Square neighborhood of West Baltimore was turned into a city museum, the H. L. Mencken House. His papers were distributed among various city and university libraries, with the largest collection held in the Mencken Room at the central branch of Baltimore's Enoch Pratt Free Library.

Mencken was born in Baltimore, Maryland, on September 12, 1880. He was the son of Anna Margaret (Abhau) and August Mencken Sr., a cigar factory owner. He was of German ancestry and spoke German in his childhood. When Henry was three, his family moved into a new home at 1524 Hollins Street facing Union Square park in the Union Square neighborhood of old West Baltimore. Apart from five years residing elsewhere during the marriage that left him a widower in middle age, Mencken would stay there for the rest of his life.

In his bestselling memoir Happy Days, he described his childhood in Baltimore as "placid, secure, uneventful and happy".

When he was nine years old, he read Mark Twain's Huckleberry Finn, which he later described as "the most stupendous event in my life". He became determined to become a writer and read voraciously. In one winter while in high school he read William Makepeace Thackeray and then "proceeded backward to Addison, Steele, Pope, Swift, Johnson and the other magnificos of the Eighteenth century". He read the entire canon of Shakespeare and became an ardent fan of Rudyard Kipling and Thomas Huxley. As a boy, Mencken also had practical interests, photography and chemistry in particular, and eventually had a home chemistry laboratory in which he performed experiments of his own design, some of them inadvertently dangerous.

He began his primary education in the mid-1880s at Professor Knapp's School on the east side of Holliday Street between East Lexington and Fayette Streets, next to the Holliday Street Theatre and across from the newly constructed Baltimore City Hall. The site today is the War Memorial and City Hall Plaza laid out in 1926 in memory of World War I dead. At 15, in June 1896, he graduated as valedictorian from the Baltimore Polytechnic Institute, at the time a males-only mathematics, technical and science-oriented public high school.