Secure Digital (SD) is a non-volatile memory card format developed by the SD Card Association for use in portable devices. It is widely used in digital cameras, cell phones, ebook readers, tablet computers, netbook computers, media players, GPS receivers, and video game consoles. The format has proven very popular and considered the the de-facto industry standard, SD technology is used by more than 400 brands across dozens of product categories and in more than 8,000 models.

SDSC (Standard-Capacity) cards, typically called SD, have an official maximum capacity of 2 GB, though commercially available up to 4 GB. SDHC (High-Capacity) cards have a maximum capacity of 32 GB, and SDXC (eXtended-Capacity) cards have a maximum capacity of 2 TB.

Changes to the interface of the established format have made some older host devices, that were designed for standard SD cards, unable to handle the newer SDHC and SDXC families. All SD card families have the same physical shape, which tends to cause confusion with consumers, thus the SD Card Association has been trying to educate the public.

In 1999, SanDisk, Matsushita, and Toshiba first agreed to develop and market the SD (Secure Digital) Memory Card, which was a development of the MMC. With a physical profile of 24×32×2.1 mm3, the new card provided both DRM up to the SDMI standard, and a high memory density for the time.

The new format was designed to compete with Sony's Memory Stick format, which was released the previous year, and featured MagicGate DRM. It was mistakenly predicted that DRM features would be widely used due to pressure from music and other media suppliers to prevent piracy.

The signature SD logo was actually developed for another use entirely: it was originally used for the Super Density Disc, which was the unsuccessful Toshiba entry in the DVD format war. This is why the D resembles an optical disc.

At the 2000 CES trade show Matsushita, SanDisk, and Toshiba Corporation announced the creation of the SD Card Association to promote SD cards. It is headquartered in California and its executive membership includes some 30 world-leading high-tech companies and major content companies. Early samples of the SD Card were available in the first quarter of 2000, with production quantities of 32 and 64 megabytes available 3 months later.

In April 2006, the SD Association released a detailed specification for the non-security related parts of the SD Memory Card standard. The organization also released specifications for the SDIO (Secure Digital Input Output) cards and the standard SD host controller. During the same year, specifications were finalized for the small-form-factor microSD (formerly known as TransFlash) and SDHC, with capacities in excess of 2 GB and a minimum sustained read/write speed of 17.6 Mb/s

In January 2009, the SD Association announced the SDXC family that will support cards up to 2 TB memory size and speeds up to 300 MBytes/sec.