The SD card is a proprietary, non-volatile, flash memory card format developed by the SD Association (SDA). They come in three physical forms: the full-size SD, the smaller miniSD (now obsolete), and the smallest, microSD. Owing to their compact form factor, SD cards have been widely adopted in a variety of portable consumer electronics, including digital cameras, camcorders, video game consoles, mobile phones, action cameras, and camera drones.

The format was introduced in August 1999 as Secure Digital by SanDisk, Panasonic (then known as Matsushita), and Kioxia (then part of Toshiba). It was designed as a successor to the MultiMediaCard (MMC) format, introducing several enhancements including a digital rights management (DRM) feature, a more durable physical casing, and a mechanical write-protect switch. These improvements, combined with strong industry support, contributed to its widespread adoption.

To manage licensing and intellectual property rights, the founding companies established SD-3C, LLC. In January 2000, they also formed the SD Association, a non-profit organization responsible for developing the SD specifications and promoting the format. As of 2023, the SDA includes approximately 1,000 member companies. The association uses trademarked logos owned by SD-3C to enforce compliance with official standards and to indicate product compatibility.

In 1994, SanDisk introduced the CompactFlash (CF) format, one of the first successful flash memory card types. CF outpaced several competing early formats, including the Miniature Card and SmartMedia. However, the late 1990s saw a proliferation of proprietary formats such as Sony's Memory Stick and the xD-Picture Card from Olympus and Fujifilm, resulting in a fragmented memory card market.

To address these challenges, SanDisk partnered with Siemens and Nokia in 1996 to develop a new postage stamp-sized memory card called the MultiMediaCard (MMC). While technically innovative, MMC adoption was slow, and even Nokia was slow to integrate support for it into its mobile devices.

In 1999, SanDisk was approached by Panasonic (then known as Matsushita) and Kioxia (then part of Toshiba) to develop a new format as a second-generation successor to MMC. The goal was to create a portable, high-performance memory card with integrated security features and broader interoperability. Concerned about losing market share to Sony's proprietary Memory Stick, Toshiba and Panasonic saw the collaboration as an opportunity to establish an open, industry-backed standard.

Panasonic and Toshiba, who had previously collaborated on the Super Density Disc (a DVD precursor), reused its stylized "SD" logo for the Secure Digital (SD) card format. Anticipating the growth of MP3 players, they also advocated for digital rights management (DRM) support seeking to reassure content publishers wary of piracy. The DRM system adopted—Content Protection for Recordable Media (CPRM)—had been developed earlier in partnership with IBM and Intel, and Intel and complied with the Secure Digital Music Initiative standard. Although often cited as a factor in the format's broad industry support, CPRM was rarely implemented in practice. SD cards also featured a mechanical write-protect switch, and early SD slots maintained backward compatibility with MMC cards.In early 2000, the first commercial SD cards offering 8 megabyte (MB) of storage were released, with larger capacity versions following shortly after. By August 2000, 64 MB cards were being sold for approximately US$200 (equivalent to $365 in 2024). According to SanDisk, consumer adoption was accelerated by Toshiba and Panasonic's commitment to launching compatible devices in parallel with the cards.

To support standardization and interoperability, SanDisk, Toshiba, and Panasonic announced the creation of the SD Association (SDA) at the January 2000 Consumer Electronics Show (CES). Headquartered in San Ramon, California, the SDA initially included 30 member companies and has since grown to encompass around 800 organizations worldwide.