A thermoplastic, also called a thermosoftening plastic is a plastic which becomes pliable or moldable above a specific temperature and returns to a solid state upon cooling.

Most thermoplastics have a high molecular weight. The polymolecules chains associate through intermolecular forces, which permits thermoplastics to be remolded because the intermolecular interactions increase upon cooling and restore the bulk properties. In this way, thermoplastics differ from thermosetting polymers, which form irreversible chemical bonds during the curing process. Thermosets often do not melt, but break down and do not reform upon cooling.

Above its glass transition temperature, Tg, and below its melting point, Tm, the physical properties of a thermoplastic change drastically without an associated phase change. Within this temperature range, most thermoplastics are rubbery due to alternating rigid crystalline and elastic amorphous regions, approximating random coils.

Some thermoplastics do not fully crystallize above glass transition temperature Tg, retaining some, or all of their amorphous characteristics. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary, as light is scattered strongly by crystallites larger than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure.

Brittleness can be decreased with the addition of plasticizers, which increases the mobility of amorphous chain segments to effectively lower Tg. Modification of the polymer through copolymerization or through the addition of non-reactive side chains to monomers before polymerization can also lower Tg. Before these techniques were employed, plastic automobile parts would often crack when exposed to cold temperatures. Recently, thermoplastic elastomers have become available.