Tsundoku (Japanese: 積ん読) is acquiring reading materials but letting them pile up in one's home without reading them.

The term originated in the Meiji era (1868–1912) as Japanese slang. It combines elements of tsunde-oku (積んでおく, to pile things up ready for later and leave) and dokusho (読書, reading books). It is also used to refer to books ready for reading later when they are on a bookshelf. As currently written, the word combines the characters for "pile up" (積) and the character for "read" (読).

The American author and bibliophile A. Edward Newton (who would not have been aware of the term) commented on a similar state.