Zuiko (Japanese: ズイコー or 瑞光) is a brand of optical lenses made by Olympus Corporation that was used up to and into the Four Thirds system era. The name Zuiko (瑞光) means 'Holy Light', using a character from the Mizuho Optic Research Laboratory (瑞穂光学研究所), where the lens was developed, and a character from Takachiho Corporation (高千穂製作所), which would eventually become the Olympus Corporation.

With the introduction of the Micro Four Thirds system in 2008, new lenses for that system started to be branded as M.Zuiko Digital.

For lenses manufactured until approximately 1972, the number of optical elements of the lens, angle of view, and diaphragm operation could be distinguished by the markings engraved on the lens. The engraving dropped the number of elements with the advent of multicoating, which occurred during the production of OM system lenses. Ace, Pen-F, FTL, and early OM system lenses carry the letter prefix denoting the number of optical elements. Later OM system lenses (generally all multicoated) omitted this prefix.

For example, an "Olympus OM-SYSTEM G.Zuiko Auto-S 1:1.4 f=50mm" is a lens with a 'Standard' focal length and a maximum aperture of f/1.4 featuring a seven-element construction and an automatic diaphragm, built for the OM-system. The use of 'standard' refers to the focal length compared to the diagonal dimension of the imager; in this case 50mm is approximately the diagonal dimension of the 35mm film frame. 'Wide-angle' lenses have focal length significantly shorter than the diagonal dimension, while 'Telephoto' lenses have focal length significantly greater than the diagonal dimension.

The first 'Zuiko'-branded lens was a 75mm f/4.5 lens fitted to the Semi-Olympus I of 1936. The Semi-Olympus used a Semi-Proud body, which took pictures in the 6×4.5 frame using medium format film.

The Olympus Flex I was a twin-lens reflex camera first sold in 1952.

In 1948, Olympus marketed the first 35mm camera in Japan, the Olympus 35 I. It was a viewfinder camera using a 24×32mm frame size fitted with a fixed (non-interchangeable) 'Zuiko Coated' 40mm f/3.5 lens; that same lens was also used on the subsequent 35 III (released in 1949 and changed the frame size to the standard 24×36mm), IV (IV in 1949, IVa in 1953, and IVb in 1954), and V (Va and Vb, both in 1955). Starting with the IV series, the lens dropped 'Coated' in favor of 'F.C.' (fully coated), in both cases referring to the anti-reflective coating.

Using a chassis similar to the 35 V, Olympus also released the Olympus Wide in 1955, a viewfinder camera featuring the D.Zuiko-W 35mm f/3.5 wide-angle lens.  A version of the Wide using the same lens was produced in 1957 with an integral uncoupled meter, branded the Wide-E.  The Wide-E was sold in the United States as the Sears Tower 54. In 1958, the Wide II viewfinder camera replaced the Wide with no change to the lens.