Multiple master fonts (or MM fonts) are an extension to Adobe Systems' Type 1 PostScript fonts, now superseded by the advent of OpenType and, in particular, the introduction of OpenType Font Variations in OpenType 1.8, also called variable fonts.

Multiple master fonts contain two or more "masters"—that is, original font styles—and enable a user to interpolate between these masters along a continuous range of "axes." With proper application support, these axes could be adjusted on demand.

The intention was that using multiple master fonts, a designer can generate a style of the exact width, thickness and optical size wanted, without losing the integrity or readability of the character glyphs. The effect is similar to morphing, as a designer can choose an intermediate between two styles, for example generating a semibold font by compromising between a bold and regular style, or perhaps extend a trend to create an ultra-light or ultra-bold. This idea was not new, having been used by companies such as URW++, but Adobe hoped to develop the technology to a greater extent.

Adobe's goal in multiple master font technology was to allow end-users of fonts to create the exact font they needed for a situation, by adjusting parameters such as boldness or width. However, multiple master fonts proved unpopular in consumer-facing use due to the difficulty of writing (or rewriting) consumer desktop publishing applications to support them, and because font designers have generally preferred to release fonts in specific weights and styles, as font files that have been individually fine-tuned. However, the multiple master concept remains heavily used at font design studios, allowing designers to generate a range of weights and styles quickly and then optimise them individually. 'Multiple master' may therefore often be seen as a generic term describing interpolated font design generally, not necessarily using Adobe technology.

In 2016, Adobe, Google, Apple and Microsoft announced a new update to the OpenType specification, allowing variable fonts. Similar to the multiple master concept, this will allow custom styles to be generated from a single font file programatically.