iDCT is an acronym for inverse Discrete Cosine Transform, a transformation step commonly used in software that works with different multimedia formats such as MP3, Vorbis, MPEG or JPEG.

One-dimensional iDCT can be thought of as moving a digitized signal from the frequency domain into the time domain, but in the case of e.g. JPEG the two-dimensional DCT makes this analogy somewhat invalid.

Several hardware implementations exist for efficient iDCT, it is a common feature of digital audio players and some graphics cards, but iDCT can also be done in software, for example mpg123, a media player for UNIX, employ efficient assembler code for the iDCT step of MP3 decoding.