High-calorie diet has been shown to impair learning ability and hippocampal synaptic plasticity in rodents. This study examined effects of daily treatment with the glucagon-like peptide-1 mimetic, exendin-4, on cognitive function and hippocampal synaptic plasticity in a model of diet-induced obesity, which exhibits compromised cognitive performance. Mice fed a high-fat diet were treated with exendin-4 (25 nmol kg−1 bodyweight; twice daily) or saline vehicle (0.9% (w/v) NaCl) over 21 days. In addition to improving metabolic control, exendin-4-treated mice exhibited a marked increase in recognition index highlighting improved learning and memory. High-fat diet resulted in the elimination of in vivo electrophysiological long-term potentiation, which was rescued following exendin-4 treatment. This study shows that exendin-4 therapy improves cognitive function and ameliorates impaired hippocampal synaptic plasticity in dietary-induced obesity.