The leishu (traditional Chinese: 類書; simplified Chinese: 类书; lit. 'category books') is a genre of reference books historically compiled in China and other East Asian countries. The term is generally translated as "encyclopedia", although the leishu are quite different from the modern notion of encyclopedia.

The leishu are composed of sometimes lengthy citations from other works, and often contain copies of entire works, not just excerpts. The works are classified by a systematic set of categories, which are further divided into subcategories. Leishu may be considered anthologies, but are encyclopedic in the sense that they may comprise the entire realm of knowledge at the time of compilation.

Approximately 600 leishu were compiled from the early third century until the eighteenth century, of which 200 have survived. The largest leishu ever compiled was the 1408 Yongle Encyclopedia, containing 370 million Chinese characters, and the largest ever printed was the Complete Classics Collection of Ancient China, containing 100 million characters and 852,408 pages.

The genre first appeared in the early third century. The earliest known was the Huanglan ("Emperor's mirror"). Sponsored by the emperor of Cao Wei, it was compiled around 220, but has since been lost. However, the term leishu was not used until the Song dynasty (960–1279).

In later imperial China dynasties, such as the Ming and Qing, emperors sponsored monumental projects to compile all known human knowledge into a single leishu, in which entire works, rather than excerpts, were copied and classified by category. The largest leishu ever compiled, on the order of the Yongle Emperor of Ming, was the Yongle Dadian containing a total of 370 million Chinese characters. The project involved 2,169 scholars, who worked for four years under general editor Yao Guangxiao. It was completed in 1408, but never printed, as the imperial treasury had run out of money.

The Complete Classics Collection of Ancient China (Qinding Gujin Tushu Jicheng) is by far the largest leishu ever printed, containing 100 million characters and 852,408 pages. It was compiled by a team of scholars led by Chen Menglei, and printed between 1726 and 1728, during the Qing dynasty.

The riyong leishu (encyclopedias for daily use), containing practical information for people who were literate but below the Confucian elite, were also compiled in the later imperial era. Today, they provide scholars with valuable information on non-elite culture and attitudes.

According to Jean-Pierre Diény, the Jiaqing reign (1796–1820) of the Qing dynasty saw the end of the publication of leishu.