Isopentenyl pyrophosphate isomerase (EC 5.3.3.2, IPP isomerase), also known as Isopentenyl-diphosphate delta isomerase, is an isomerase that catalyzes the conversion of the relatively un-reactive isopentenyl pyrophosphate (IPP) to the more-reactive electrophile dimethylallyl pyrophosphate (DMAPP). This isomerization is a key step in the biosynthesis of isoprenoids through the mevalonate pathway and the MEP pathway.

isopentenyl diphosphate ${\displaystyle \rightleftharpoons }$ dimethylallyl diphosphate

This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases transposing C=C bonds. The systematic name of this enzyme class is isopentenyl-diphosphate Delta3-Delta2-isomerase. Other names in common use include isopentenylpyrophosphate Delta-isomerase, methylbutenylpyrophosphate isomerase, and isopentenylpyrophosphate isomerase.