Polyetherimide (PEI; branded as Ultem) is an amorphous, amber-to-transparent thermoplastic with characteristics similar to the related plastic PEEK. When comparing PEI to PEEK, the former is cheaper but has lower impact strength and a tighter temperature range.

PEI plastics were first introduced into the market by General Electric (GE) in 1982 under the trade name Ultem resulting from the work of J.G. Wirth's research team in the early 1970s.

Due to its adhesive properties and chemical stability it became a popular bed material for FFF 3D printers.

The molecular formula of the PEI repeating unit is C₃₇H₂₄O₆N₂ and the molecular weight is 592.61 g/mol. It contains phthalimide and bisphenol A sub-units.

The glass transition temperature of PEI is 217 °C (422 °F). Its amorphous density at 25 °C is 1.27 g/cm³(.046 lb/in³). It is prone to stress cracking in chlorinated solvents. Polyetherimide is able to resist high temperatures while maintaining stable electrical properties over a wide range of frequencies. This high strength material offers excellent chemical resistance and ductile properties suitable for various applications, even those involving steam exposure.

PEIs are manufactured by the imidization reaction of a flexible dianhydride and m-Phenylenediamine.