The Portsmouth Athenæum is a non-profit membership library and museum, incorporated in 1817 and located in the heart of historic Portsmouth, New Hampshire. Membership libraries were first created in the 18th century for the mutual edification of their members and to elevate the educational resources available in the community. The name "Athenæum" was used by a number of societies established during the nineteenth century to describe institutions with broader aims than just a library. The term is derived from Athena, the Greek goddess of wisdom, and the classical temple of the arts and sciences named to honor her.

While there were once hundreds of membership libraries across America, there are now fewer than twenty left. The Athenæum today continues this long tradition of mutual improvement by maintaining a library of over 40,000 volumes and an archive of manuscripts, photographs, objects, and ephemera relating to local history. It also sponsors exhibitions, concerts, lectures, and other educational and cultural programs.