联合目录是指一种联合二所以上图书馆馆藏目录的数据库。用户从单一窗口网站来检索国内多所图书馆的馆藏，其能提供给用户知道哪个图书馆有收藏他所需要的馆藏信息。

目前全球最大的在线联合目录为OCLC所提供的「WorldCat」数据库；目前国内最大的在线联合目录为「全国图书书目信息网」（National Bibliographic Information Network，简称NBINet），它是由参与合作图书馆和国家图书馆所共同合作编目和维护的书目数据库。

A union catalog is a combined library catalog describing the collections of a number of libraries. Union catalogs have been created in a range of media, including book format, microform, cards and more recently, networked electronic databases. Print union catalogs are typically arranged by title, author or subject (often employing a controlled vocabulary); electronic versions typically support keyword and Boolean queries.