The Family Computer Disk System, commonly shortened to the Famicom Disk System, is a peripheral for Nintendo's Family Computer (Famicom) home video game console, released in Japan on February 21, 1986. The system uses proprietary floppy disks called "Disk Cards" for more affordable data storage and adds a high-fidelity sound channel to enhance audio in compatible Disk System games.

To support the Disk System, Nintendo installed "Disk Writer" kiosks in stores across Japan that allowed customers to bring their Disk Cards and have new games rewritten onto them for a small fee, making it a cost-effective alternative to purchasing games on traditional RAM cartridges. Nintendo also offered similar disk rewriting services by mail.

The Disk System was designed to enhance features already present in the base Famicom, offering better sound and cheaper, rewritable games. However, it came with drawbacks, including a high initial price for the device along with the storage medium's slower load times and reduced reliability. Despite these limitations, the Disk System’s rewritable storage served as an enabling technology, enabling the creation of new types of video games. This era saw the rise of expansive open world adventures like The Legend of Zelda (1986) and Metroid (1986) enabled by progress-saving, games with cost-effective and swift releases such as the best-selling Super Mario Bros. 2, and nationwide leaderboards and contests via in-store Disk Fax kiosks, which are considered to be forerunners of today's online achievement and distribution systems.

While sales of the Disk System peripheral ended in 1993, after selling 4.4 million units, making it the most successful console add-on of all time, support for the system continued well beyond that point. The final title for the Disk System was Janken Disk Jo, released in December 1992. Nintendo maintained Disk writing services by mail until 2003, and provided technical support until 2007.

The Family Computer was an instant success for Nintendo. By January 1985, eighteen months after its launch, over three million units had been sold, and Nintendo was in command of the Japanese home video game market. This success came with drawbacks. It was difficult to meet demands for stock from retailers, sometimes because of chip shortages, and retailers had been requesting cheaper games than those on the chip-based cartridges used by the Famicom. Consequently, Nintendo decided to investigate how to lower the cost of games.

In July 1983, Nintendo had rejected a proposal by Hudson Soft for a Famicom add-on which used Bee Cards, ROM-based cartridges tested on their MSX computer. Though this allowed for the saving of games, the technology was expensive, and royalties would be paid for each card sold. Nintendo remembered this during the research process for low-cost games, striking on releasing games on a medium similar to floppy disks, quickly becoming the standard medium for storage on personal computers. Not only were they cheap to produce, their increased capacity compared to cartridges enabled longer games, improved music and sound effects, and rewritable saves. Nintendo's proprietary platform was named the Disk Card, and based on Mitsumi's Quick Disk, a cheaper alternative to floppy disks for Japanese home computers.

Disk Cards were to be used with an add-on to the Famicom, the Famicom Disk System. This was produced by Masayuki Uemura and Nintendo Research & Development 2, the same team which designed the Famicom. Following several delays, it was released in Japan on February 21, 1986, at a retail price of ¥15000 (US$80). Launch titles included The Legend of Zelda, and re-releases of earlier Famicom games. Aided by marketing material featuring a yellow mascot character named Diskun, or Mr. Disk, the FDS sold over 300,000 units within its first three months on the market, a number which had jumped to over 2 million by the end of the year. Nintendo had great confidence in the FDS, resolving to make future first-party releases exclusive to the system, installing Disk Writers, kiosks from which consumers could download games onto their disks, in toy and electronic stores, and introducing high-score tournaments for specific Disk System games, where players could submit their scores directly to Nintendo via Disk Fax machines found in retail stores. Winners would receive exclusive prizes, including Famicom-branded stationery sets and a gold-colored Punch-Out!! cartridge.

Despite the Famicom Disk System's success, and advantages over the Famicom itself, it also imposed many problems of its own. Most common was the quality of the Disk Cards; not only were they fragile, because the shutters on most disks were removed to reduce costs, it was common for them to collect fingerprints and dust, eventually rendering their games unplayable. Piracy was rampant, with disk copying devices and bootleg games becoming commonplace in stores and in magazine advertisements. Third-party developers were angered by Nintendo's requirement that it own half of the copyright of FDS games, with several, including Namco and Hudson Soft, refusing to produce games for the system. Capcom released a port of Ghosts 'n Goblins with a 128KB ROM size, proving that it was possible to publish FDS-size games on the Famicom. Retailers disliked the Disk Writer kiosks for taking up too much space and for generally being unprofitable. The Disk System's vague error messages, long loading times, and the poor quality of the rubber drive belt that spun the disks are also cited as attributing to its downfall. For these reasons, a planned American release, announced in 1986, had been cancelled by November 1988, when it was approved by the Japanese branch.