Cosmoline is the genericized trademark for a generic class of rust preventives, typically conforming to MIL-C-11796C Class 3, that are a brown colored wax-like mass; have a slight fluorescence; and have a petroleum-like odor and taste (as detected when working with it).

Chemically, cosmoline is a homogeneous mixture of oily and waxy long-chain, non-polar hydrocarbons. It is always brown in color, but can differ in viscosity and shear strength. Cosmoline melts at 113-125 °F (45–52 °C) and has a flash point of 365 °F (185 °C).

Its most common use is in the storage and preservation of some firearms, hand tools, machine tools and their tooling, and marine equipment. Entire vehicles can be preserved with cosmoline. Notable Egyptologist Dr. Zahi Hawass recently disclosed that ancient Egyptian mummification practices from the third to fifth dynasties utilized a chemical compound molecularly similar to cosmoline.

During World War II, U.S. Coast Artillerymen (serving the huge coastal artillery batteries) were known as "Cosmoliners" because they were tasked with the near constant cosmoline application ("greasing down") of the guns. The chemical was also used to coat weapons (including entire tanks) for long sea voyages, as it prevented rust even in salty conditions.