Microfiber or microfibre is synthetic fiber finer than one denier or decitex/thread. This is smaller than the diameter of a strand of silk (which is approximately one denier), which is itself about 1/5 the diameter of a human hair. The most common types of microfibers are made from polyesters, polyamides (e.g., nylon, Kevlar, Nomex, trogamide), or a conjugation of polyester, polyamide, and polypropylene (Prolen).

Microfiber is used to make mats, knits, and weaves for apparel, upholstery, industrial filters, and cleaning products. The shape, size, and combinations of synthetic fibers are selected for specific characteristics, including softness, toughness, absorption, water repellency, electrostatics, and filtering capabilities.

Production of ultra-fine fibers (finer than 0.7 denier) dates back to the late 1950s, using melt-blown spinning and flash spinning techniques. However, only fine staples of random length could be manufactured and very few applications could be found. Experiments to produce ultra-fine fibers of a continuous filament type were made subsequently, the most promising of which were run in Japan during the 1960s by Dr. Miyoshi Okamoto, a scientist at Toray Industries. Okamoto's discoveries, together with those of Dr. Toyohiko Hikota, resulted in many industrial applications.

Among these was Ultrasuede, one of the first successful synthetic microfibers, which found its way onto the market in the 1970s. Microfiber's use in the textile industry then expanded. Microfibers were first publicized in the early 1990s in Sweden and saw success as a product in Europe over the course of the decade.

In cleaning products, microfiber can be 100% polyester, or a blend of polyester and polyamide (nylon). It can be both a woven product or a non woven product, the latter most often used in limited use or disposable cloths.In the highest-quality fabrics for cleaning applications, the fiber is split during the manufacturing process to produce multi-stranded fibres. A cross section of the split microfiber fabric under high magnification would look like an asterisk. The split fibres and the size of the individual filaments working in conjunction with the spaces between them that make the cloths more effective than other fabrics for cleaning purposes. The structure traps and retains the dirt and also absorbs liquids.

Unlike cotton, microfiber leaves no lint, the exception being some micro suede blends, where the surface is mechanically processed to produce a soft plush feel.

For microfiber to be most effective as a cleaning product, especially for water-soluble soils and waxes, it should be a split microfiber. Non-split microfiber is little more than a very soft cloth. The main exception is for cloths used for facial cleansing and for the removal of skin oils, (sebum), sunscreens, and mosquito repellents from optical surfaces such as cameras, phones and eyeglasses where in higher-end proprietary woven, 100% polyester cloths using filaments, will absorb these types of oils without smearing.

Microfiber that is used in non-sports-related clothing, furniture, and other applications isn't split because it isn't designed to be absorbent, just soft. When buying, microfiber may not be labeled to designate whether it is split. A quick way to determine if microfiber is, is to run the cloth lightly over the palm of the hand. A split microfiber will Cling to any imperfections of the skin, which can be both heard and felt. Another way is to pour a small amount of water on a hard flat surface and try to push the water with the microfiber. If the water is pushed rather than being absorbed, it's not split microfiber.

Microfiber can be electrostatically-charged for special purposes like filtration.