A mimeograph machine (often abbreviated to mimeo, sometimes called a stencil duplicator) is a low-cost duplicating machine that works by forcing ink through a stencil onto paper. The process is called mimeography, and a copy made by the process is a mimeograph.

Mimeographs, along with spirit duplicators and hectographs, were common technologies for printing small quantities of a document, as in office work, classroom materials, and church bulletins. Early fanzines were printed by mimeograph because the machines and supplies were widely available and inexpensive. Beginning in the late 1960s and continuing into the 1970s, photocopying gradually displaced mimeographs, spirit duplicators, and hectographs.

For even smaller quantities, up to about five, a typist would use carbon paper.