xD-Picture Card is an obsolete flash memory card format, developed jointly by Olympus and Fujifilm in 2002 as a proprietary alternative to existing formats. It was primarily used in digital cameras produced by Olympus and Fujifilm, and was also adopted by Kodak in some models. xD cards were available in capacities ranging from 16 MB to 2 GB. While their file transfer speeds were good by 2002 standards, by the late 2000s they offered less capacity, were slower, and were more expensive than SD cards. The format was phased out in 2009, when Fujifilm and Olympus transitioned to the more widely supported SD card format.

The cards were developed by Olympus and Fujifilm, and introduced into the market in July 2002. Toshiba Corporation and Samsung Electronics manufactured the cards for Olympus and Fujifilm. xD cards were sold under other brands, including Kodak, SanDisk, PNY, and Lexar, but were not branded with the respective companies' logos, except for Kodak. The name of the card was "inspired by" the phrase "eXtreme digital". xD cards competed primarily with Secure Digital (SD) cards, CompactFlash (CF), and Sony's Memory Stick. On its launch, some people anticipated that xD cards would become the new standard memory card format. Because of its higher cost and limited usage in products other than digital cameras, xD lost ground to SD, which is broadly used by cellular phones, personal computers, digital audio players and many other digital cameras.

Olympus included a bundled adaptor (MASD-1) with their cameras which allowed the use of microSD cards beginning with the FE-360 and FE-370 in August 2008. These adaptors were included with, and are only compatible with, Olympus cameras released between August 2008 and August 2009. Fuji did not release any such adaptor for use with their cameras, however they did release cameras which were compatible with both xD and SD cards, such as the FinePix Z10fd. The MASD-1 adaptor only physically fits in compatible Olympus cameras. Forcing the adapter into an incompatible camera can damage the XD card slot. Some compatible cameras must have their firmware upgraded to version 1.1 or above in order to use the adaptor. Only SanDisk and Toshiba microSD cards from 256 MB to 8 GB in capacity were officially supported.

Olympus began to move away from the xD format with the mid-2009 announcement of the E-P1 camera, which supported only Secure Digital memory cards. Higher-end DSLR cameras such as the E-3 and E-5 among others continued to use CompactFlash cards as well. The last Fujifilm camera which accepted xD cards was the Fujifilm FinePix F200EXR which was announced in February 2009. This camera also accepted SD cards. The final Olympus cameras to use xD-Picture Cards were the FE-5020, FE-4010, FE-4000, FE-46 and FE-26, which were all released in August 2009. xD cards were discontinued around the same time; Amazon Best Sellers in xD-Picture Cards reports no products offered with a Date First Available since 4 August 2009. The abandonment of the xD-Picture Card format and changeover to the SD card format was not officially acknowledged by Olympus or FujiFilm.

Four types of xD cards were produced over the format's lifetime. The first type had no type designation printed on the card but were retrospectively called Standard or Type S cards once other variants entered the market. Type M cards used multi-level cell technology to boost capacity at the expense of speed. Type H (high speed) cards had the fastest write speeds of any xD card but were discontinued in 2008 due to high manufacturing costs. The final type, M+ (which stood for enhanced type M) was introduced one year before Olympus and Fujifilm abandoned the format. There were plans to make xD-Card drives of up to 8 GB capacity, however this was never achieved.

Olympus claimed that its xD cards supported special "picture effects" when used in some Olympus cameras, though these software features are not intrinsically hardware-dependent. Type H and M+ cards however, are required in later models to capture 640×480 video at 30 FPS. Due to changes in the cards' storage architecture, later Type M and H cards may have compatibility issues with some older cameras (especially video recording). Compatibility lists are available for Olympus and Fujifilm cameras.

Detailed specifications are tightly controlled by Olympus and Fujifilm, which charge licensing fees and royalties and require non-disclosure agreements in exchange for the technical information required to produce xD-compatible devices.

The memory format used is not well-documented. It is difficult to study it directly, since most camera devices and most USB card readers do not provide direct access to the flash memory. Since the cards are controller-less, cameras and card readers must perform wear leveling and error detection. They normally hide the portion of the memory which stores this information (among other things) from higher level access.