Glutamate, one of the most important central excitatory neurotransmitters, plays crucial roles in nerve signal transduction and is implicated in several neurological disorders. However, no effective means has been developed for specific detection of glutamate released from primary cultured neurons. Here we present a reduced graphene oxide (RGO)-based field effect transistor (FET) biosensor functionalized with synthesized glutamate receptor for real-time monitoring of glutamate release from primary cultured rat hippocampus neurons. Metabotropic glutamate receptors (mGluR) was specifically synthesized and then immobilized on the RGO surface by 1-pyrenebutanoic acid succinimidyl ester (PASE) linker, after which target glutamate (pI = 3.22) could specifically bind to the synthesized mGluR in the neutral buffer, causing the charge density change. After the neurons were cultured on the sensing channel with a self-made liquid reservoir, the FET biosensor could discriminate glutamate in the femtomolar range in complete cell culture medium and generate encouraging results in real-time monitoring of glutamate release from primary rat hippocampus neurons. This work is the first report of specific and direct detection of glutamate molecules released from primary culture of differentiated central neurons, which may further help understand the nature of neuronal communication. Moreover, this work paves a way for the detection of electrochemically inactive small molecules released by cells.