Pathogenic variants in GABAA receptor subunits are a prominent cause of genetic epilepsies. Recent biophysical analysis of pathogenic GABRB3 variants reveals that both loss- and gain-of-function can cause epilepsy. Stratification shows that patients carrying gain-of-function variants experience an earlier onset of seizures that are refractory and have worse developmental delay compared to loss-of-function variant patients. This suggests that distinct pathological mechanisms underlie disease in each GABRB3 functional class. To date, research has only focused on loss-of-function variants. Here, we engineer a mouse model based on the human de novo gain-of-function GABRB3 p.E77K variant associated with a developmental and epileptic encephalopathy with epileptic spasms, myoclonic seizures, hypotonia, severe developmental delay and autism spectrum disorder. Gabrb3E77K mice exhibited non-Mendelian ratios at birth, suggesting a high rate of mortality either in utero or shortly after birth. Surviving mice continued to die prematurely. Although spontaneous seizures were not observed, Gabrb3E77K mice were more susceptible to a pro-convulsant challenge. ECoG findings showed alterations to brain activity with a clear increase in overall power spectrum. Behavioural assays indicated Gabrb3E77K mice had decreased locomotion and decreased anxiety. Voltage clamp recordings from Gabrb3E77K CA1 hippocampal pyramidal neurons revealed larger spontaneous inhibitory post-synaptic currents consistent with gain-of-function findings from recordings in Xenopus oocytes. The Gabrb3E77K mouse confirms that gain-of-function GABRB3 variants can enhance inhibitory synaptic transmission and lead to epilepsy, providing an opportunity to understand neuronal network mechanisms. The mouse also provides a valuable pre-clinical model for developing targeted interventions and therapeutic strategies for individuals affected by this disease.