Indium is a chemical element with chemical symbol In and atomic number 49. This rare, soft, malleable and easily fusible poor metal is chemically similar to aluminium or gallium but more closely resembles zinc (zinc ores are also the primary source of this metal). Its current primary application is to form transparent electrodes from indium tin oxide in liquid crystal displays. It is widely used in thin-films to form lubricated layers (during World War II it was widely used to coat bearings in high-performance aircraft). It's also used for making particularly low melting point alloys, and is a component in some lead-free solders.

Indium is a very soft, silvery-white, relatively rare true metal with a bright luster. As a pure metal indium emits a high-pitched "cry", when it is bent. Both gallium and indium are able to wet glass.

One unusual property of indium is that its most common isotope is slightly radioactive; it very slowly decays by beta emission to tin. This radioactivity is not considered hazardous, mainly because its half-life is 4.41×1014 years, four orders of magnitude larger than the age of the universe and nearly 50,000 times longer than that of natural thorium. Unlike its period 5 neighbor cadmium, indium is not a notorious cumulative poison.