Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

Many kinds of polyethylene are known, with most having the chemical formula (C₂H₄) PE is usually a mixture of similar polymers of ethylene, with various values of n. It can be low-density or high-density: low-density polyethylene is extruded using high pressure (1,000–5,000 atm (100–510 MPa)) and high temperature (520 K (247 °C; 476 °F)), while high-density polyethylene is extruded using low pressure (6–7 atm (610–710 kPa)) and low temperature (333–343 K (60–70 °C; 140–158 °F)). Polyethylene is usually thermoplastic, but it can be modified to become thermosetting instead, for example, in cross-linked polyethylene. While monomers with an -ene suffix have a double bonded carbon pair, that double bond is lost in the polymerization of polyethylene; the -ethylene is retained in the name only to reflect the monomer form from which it is polymerized.