S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable using RCA or BNC connectors, or a fibre-optic cable using TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high-fidelity systems.

S/PDIF is based on the AES3 interconnect standard. S/PDIF can carry two channels of uncompressed PCM audio or compressed 5.1 surround sound; it cannot support lossless surround formats that require greater bandwidth.

S/PDIF is a data link layer protocol as well as a set of physical layer specifications for carrying digital audio signals over either optical or electrical cable. The name stands for Sony/Philips Digital Interconnect Format but is also known as Sony/Philips Digital Interface. Sony and Philips were the primary designers of S/PDIF. S/PDIF is standardized in IEC 60958 as IEC 60958 type II (IEC 958 before 1998).

A common use is to carry two channels of uncompressed digital audio from a CD player to an amplifying receiver.

The S/PDIF interface is also used to carry compressed digital audio for surround sound as defined by the IEC 61937 standard. This mode is used to connect the output of a Blu-ray, DVD player or computer, via optical or coax, to a home theatre amplifying receiver that supports Dolby Digital or DTS Digital Surround decoding.

S/PDIF was developed at the same time as the main standard, AES3, used to interconnect professional audio equipment in the professional audio field. This resulted from the desire of the various stakeholders to have at least sufficient similarities between the two interfaces to allow the use of the same, or very similar, designs for interfacing ICs. S/PDIF is nearly identical at the protocol level, but uses either coaxial cable (with RCA connectors) or optical fibre (TOSLINK; i.e., JIS F05 or EIAJ optical), both of which cost less than the XLR connection used by AES3. The RCA connectors are typically colour-coded orange to differentiate from other RCA connector uses such as composite video. S/PDIF uses 75 Ω coaxial cable while AES3 uses 110 Ω balanced twisted pair.

Signals transmitted over consumer-grade TOSLINK connections are identical in content to those transmitted over coaxial connectors. Optical provides electrical isolation that can help address ground loop issues in systems. The electrical connection can be more robust and supports longer connections.

S/PDIF is used to transmit digital signals in a number of formats, the most common being the 48 kHz sample rate format (used in Digital Audio Tape and DVDs) and the 44.1 kHz format, used in CD audio. In order to support both sample rates, as well as others that might be needed, the format has no defined bit rate. Instead, the data is sent using biphase mark code, which has either one or two transitions for every bit, allowing the original word clock to be extracted from the signal itself.