Secure Digital, officially abbreviated as SD, is a proprietary, non-volatile, flash memory card format the SD Association (SDA) developed for use in portable devices.

Because of their small physical dimensions, SD cards became widely used in many consumer electronic devices, such as digital cameras, camcorders, video game consoles, mobile phones, action cameras such as the GoPro Hero series, and camera drones.

The standard was introduced in August 1999 by SanDisk, Panasonic (Matsushita) and Toshiba as an improvement on MultiMediaCards (MMCs). SDs have become an industry standard. The three companies formed SD-3C, LLC, a company that licenses and enforces intellectual property (IP) rights associated with SD memory cards and SD host-and-ancillary products.

In January 2000, the companies formed the SD Association (SDA), a non-profit organization to create and promote SD Card standards. As of 2023, the SDA has approximately 1,000 member companies. It uses several SD-3C-owned trademarked logos to enforce compliance with its specifications and denote compatibility.