Immediate early genes (IEGs) are genes which are activated transiently and rapidly in response to a wide variety of cellular stimuli. They represent a standing response mechanism that is activated at the transcription level in the first round of response to stimuli, before any new proteins are synthesized. IEGs are distinct from "late response" genes, which can only be activated later, following the synthesis of early response gene products. Thus IEGs have been called the "gateway to the genomic response". The term can describe viral regulatory proteins that are synthesized following viral infection of a host cell, or cellular proteins that are made immediately following stimulation of a resting cell by extracellular signals.

In their role as "gateways to genomic response", many IEG products are natural transcription factors or other DNA-binding proteins. However, other important classes of IEG products include secreted proteins, cytoskeletal proteins, and receptor subunits. Neuronal IEGs are used prevalently as a marker to track brain activities in the context of memory formation and development of psychiatric disorders. IEGs are also of interest as a therapeutic target for treatment of human cytomegalovirus.