The Four Thirds System is a standard created by Olympus and Eastman Kodak for digital single-lens reflex camera (DSLR) design and development. Four Thirds refers to both the size of the image sensor (4/3") as well as the aspect ratio (4:3). The Olympus E-1 was the first Four Thirds DSLR, announced and released in 2003. In 2008, Olympus and Panasonic began publicizing the Micro Four Thirds system, a mirrorless camera system which used the same sensor size; by eliminating the reflex mirror, the Micro Four Thirds cameras were significantly smaller than the Four Thirds cameras. The first Micro Four Thirds cameras were released in 2009 and the final Four Thirds cameras were released in 2010; by that time, approximately 15 Four Thirds camera models had been released by Olympus and Panasonic in total.

The system provides a standard that permits interoperability of digital cameras and lenses made by different manufacturers. Proponents describe it as an open standard, but companies may use it only under a non-disclosure agreement.

Unlike older single-lens reflex (SLR) systems, Four Thirds was designed from the start for digital cameras. Many lenses are extensively computerised, to the point that Olympus offers firmware updates for many of them. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs.

The image sensor format, between those of larger SLRs using "full-frame" and APS-C sensors, and smaller point-and-shoot compact digital cameras, yields intermediate levels of cost, performance, and convenience. The size of the sensor is smaller than most DSLRs and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal length would cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard, and is correspondingly more compact. Thus, the Four Thirds System has crop factor (aka focal length multiplier) of about 2, and while this enables longer focal length for greater magnification, it does not necessarily aid the manufacture of wide angle lenses.

Kodak and Olympus announced in February 2001 they would share digital camera technologies; Olympus committed to purchase high-resolution charge-coupled device (CCD) sensors which would be jointly developed by the two companies and manufactured by Kodak. A few months later, an internal Kodak presentation revealed that Olympus was developing a DSLR using Kodak's KAF-C5100E 5.1 megapixel 4/3" sensor, with a tentative schedule to announce the camera at the 2002 Photo Marketing Association exposition; Olympus confirmed they were developing a "concept camera" with that sensor size.

The Four Thirds System was announced jointly by Olympus and Kodak at photokina in September 2002. The first camera was the Olympus E-1, announced on June 24, 2003, and aimed at the professional market, with shipments to commence in September. In February 2004, Olympus announced that Panasonic, Sanyo, and Sigma Corporation had joined the consortium. The second Four Thirds DSLR, the Olympus E-300, was introduced that year, without the typical protrusion on the top deck, as the designers had chosen to use a "porro finder" which had four mirrors instead of a standard pentaprism, similar to the design of the viewfinder used in the Olympus Pen F half-frame film SLR.

In 2006, Olympus and Panasonic announced they had collaborated on the design of a new sensor, branded Live MOS, using a body design similar to that of the E-300; the result was three similar cameras, sold as the Olympus E-330, Panasonic DMC-L1, and Leica Digilux 3. Nearly all of the successive Four Thirds camera models would use sensors from Panasonic, with the sole exception of the Olympus E-400 (2006), which was equipped with a CCD but sold only in Europe.

In August 2008, Olympus and Panasonic introduced a new format, Micro Four Thirds.