The National Puzzlers' League (NPL) is a nonprofit organization focused on puzzling, primarily in the realm of word play and word games. Founded in 1883, it is the oldest puzzlers' organization in the world. It originally hosted semiannual conventions in February and September of each year, but conventions are now held annually, in July.

On July 4, 1883, twenty-eight word puzzlers, mostly young men, met at Pythagoras Hall in New York City and founded the Eastern Puzzlers' League; they then celebrated the event by paying a penny each to walk across the newly dedicated Brooklyn Bridge. Renamed the National Puzzlers' League in 1920, the organization has been in continuous existence ever since that first meeting and is the oldest puzzlers' organization in the world.

The league's official publication began as The Eastern Enigma. It originally contained few, if any, puzzles, and instead reported on business transacted at puzzlers' conventions, presented verses and skits composed by members, and relayed debates on the controversial puzzling topics of the day, such as obsolete words, esoteric references, and new types of puzzles.

Early editors rarely served more than a year at a time, and the publication schedule was often irregular. The January 1900 issue initiated a puzzle department called "Penetralia", which appeared regularly until 1903. "Penetralia" was resumed in the February 1910 issue, when The Eastern Enigma began monthly publication, and has continued to appear ever since. On the organization's name change, the magazine was retitled The Enigma; it continues to be published monthly and has now reached over 1200 issues.

A puzzle renaissance in the 1920s and 1930s led to a high point in NPL membership, during which it adopted the slogan "The National Intellectual Pastime of America". In 1935, president Everett M. Smith invented the word Pneumonoultramicroscopicsilicovolcanoconiosis at the annual League meeting. The biennial conventions were covered by major newspapers and wire services. During World War II, the Signal Intelligence Service recruited several NPL members as government cryptographers based on their puzzling experience.

In the 1960s, membership slumped significantly, but began rebounding in the "modern" puzzling era with the promotional efforts of Dmitri Borgmann, Ross Eckler, and others. Many of the leading puzzle editors and constructors in America are members; author and member Willard Espy said of the league's expertise, "I'm a little nervous talking about wordplay with this group .... it's like talking about religion with God."

Each NPL member generally chooses a "nom" (short for "nom de plume") upon joining, an often cryptic pen name which serves as a nickname when communicating with fellow members (see below for an example). The use of pseudonyms, also commonly practiced by the compilers of cryptic crosswords, originally helped "to break down barriers of occupation or social class". It is also an opportunity for wordplay and self-description.

NPL members are known collectively as "the Krewe" and individually as "puzzlers", "Krewe members", or "NPLers". As of 2018 the organization had about 700 members, of whom about 200 attend the group's convention each year.