A marker pen, fine liner, marking pen, felt-tip pen, felt pen, flow marker, sign pen (in South Korea), vivid (in New Zealand), flomaster (in East and South Slavic countries), texta (in Australia), sketch pen (in South Asia), koki (in South Africa) or simply marker is a pen which has its own ink source and a tip made of porous, pressed fibers such as felt. A marker pen consists of a container (glass, aluminum or plastic) and a core of an absorbent material that holds the ink. The upper part of the marker contains the nib that was made in earlier times of a hard felt material, and a cap to prevent the marker from drying out.

Until the early 1990s, the most common solvents that were used for the ink in permanent markers were toluene and xylene. These two substances are both harmful and characterized by a very strong smell. Today, the ink is usually made on the basis of alcohols (e.g. 1-Propanol, 1-butanol, diacetone alcohol and cresols).

Markers may be waterproof, dry-erase, wet-erase (e.g. transparency markers), or permanent.

Lee Newman patented a felt-tipped marking pen in 1910. In 1926, Benjamin Paskach patented a "fountain paintbrush", as he called it, which consisted of a sponge-tipped handle containing various paint colors. Markers of this sort began to be popularized with the sale of Sidney Rosenthal's Magic Marker (1953), which consisted of a glass tube of ink with a felt wick. By 1958, use of felt-tipped markers was commonplace for a variety of applications such as lettering, labeling, and creating posters. The year 1962 brought the development of the modern fiber-tipped pen (in contrast to the marker, which generally has a thicker point) by Yukio Horie of the Tokyo Stationery Company (which later became Pentel).

In the 1980s, the first security markers were introduced, which use invisible but fluorescent ink. This ink can be used to mark valuable objects, and in the event of a theft, these markings can be detected with ultraviolet light. In 1987 Copic Sketch markers were released, further popularising markers for professional illustration.

The marker reservoir, which holds the ink, is formed from polyester. The "felt" used for the tip is usually made of highly compressed synthetic fibers or porous ceramics. Toluol and xylol were used as solvents for the dye and are still used for the indelible ink in permanent markers. Due to their toxicity, they have often been replaced with less critical substances such as alkyl or cyclic alkylene carbonates (like propylene carbonate) in other types of markers. Water content of the ink can be up to 10%. Besides solvents and the dye itself, the ink may contain additives (e.g. nonylphenylpolyglycol ether, alkylpoly-glycol ether, fatty acid polyglycol ester, or fatty alcohol ethoxalates) and preservatives (e.g. 2-Phenylphenol and its sodium salt, 6-acetoxy-2,4-dimethyl-m-dioxane).

Permanent markers are porous pens that can write on surfaces such as glass, plastic, wood, metal, and stone. On most surfaces, the ink is generally resistant to rubbing and water, and can last for many years. However, on certain plastics like Teflon, polypropylene etc., the marks made by such pens are not permanent and can be erased easily. Depending on the surface and the marker used, the marks can often be removed with either vigorous scrubbing or chemicals such as acetone.

Highlighters are a form of marker used to highlight and cover over existing writing while still leaving the writing readable. They are generally produced in neon colours to allow for colour coding, as well as attract buyers to them. As early as 1963, the American company Carter's Ink Company developed fluorescent felt-tip pens. Felt-tip pens with fluorescent ink were first developed and marketed in Germany in 1971 by Günter Schwanhäußer of Schwan-Stabilo.