LibraryThing is a social cataloging web application for storing and sharing book catalogs and various types of book metadata. It is used by individuals, authors, libraries and publishers.

Based in Portland, Maine, LibraryThing was developed by Tim Spalding and went live on August 29, 2005. As of May 2012 it has over 1,530,000 users and more than 72 million books catalogued.

The primary feature of LibraryThing is the cataloging of books by importing data from libraries through Z39.50 connections and from six Amazon.com stores. Library sources supply MARC and Dublin Core records to LT; users can import information from 690 libraries, including the Library of Congress, National Library of Australia, the Canadian National Catalogue, the British Library, and Yale University. Should a record not be available from any of these sources, it is also possible to add the book information by using a blank form.

LibraryThing's social features have been compared to bookmark manager Del.icio.us and the collaborative music service Last.fm. Similar book cataloging sites include Goodreads, Shelfari, aNobii, BookJetty, and weRead.