An excitatory synapse is a synapse in which an action potential in a presynaptic neuron depolarizes the membrane of the postsynaptic cell, and thus increases the probability of triggering an action potential in that cell. The postsynaptic cell—a muscle cell, a glandular cell or another neuron—typically receives input signals through many excitatory and many inhibitory synapses. If the total of excitatory influences exceeds that of the inhibitory influences and the resulting depolarization exceeds the threshold level, the postsynaptic cell will be activated. If the postsynaptic cell is a neuron it will generate a new action potential at its axon hillock, thus transmitting the information to yet another cell. If it is a muscle cell, it will contract. If it is a gland cell, it will release its product (e.g., hormone).

In an excitatory synapse, the electric response of the postsynaptic membrane to a single action potential in the presynaptic neuron is known as an excitatory postsynaptic potential (EPSP). It may occur via direct contact between cells (i.e., via gap junctions), as in an electrical synapse, but most commonly occurs via the vesicular release of neurotransmitters from the presynaptic axon terminal into the synaptic cleft, as in a chemical synapse.

The excitatory neurotransmitters, the most common of which is glutamate, then migrate via diffusion to the dendritic spine of the postsynaptic neuron and bind a specific transmembrane receptor protein that triggers the depolarization of that cell. Depolarization, a deviation from a neuron's resting membrane potential towards its threshold potential, increases the likelihood of an action potential and normally occurs with the influx of positively charged sodium (Na⁺) ions into the postsynaptic cell through ion channels activated by neurotransmitter binding.