Arsenic exposure through drinking water can impair the learning and memory ability of children in China and other countries. Synaptic plasticity plays a key role in the process of learning and memory. Alterations in the expression of presynaptic and postsynaptic proteins can be used to evaluate synaptic plasticity, and further to evaluate impairment in learning and memory ability. Thereby, the aim of this study was to explore the mechanisms underlying arsenic neurotoxicity by focusing on alterations in the hippocampal synapses of mouse offspring induced by developmental arsenite exposure. Mother mice and their offspring were exposed to 0, 25, 50 or 100 mg L arsenite via drinking water from the first day of gestation until postnatal day (PND) 35. The spatial learning and memory ability of PND 35 mice was evaluated using a Morris water maze. The levels of speciated arsenicals in the brain of PND 7, 14, 21 and 35 mice were analyzed by hydride generation coupled with atomic absorption spectrophotometry. Synaptic structure and protein expression of postsynaptic density protein-95 (PSD-95) and synaptophysin (SYP) in the hippocampus of PND 7, 14, 21 and 35 mice were examined. The findings from this study disclosed that the spatial learning ability of mice could be impaired by exposure to 25 mg L arsenite; however spatial memory ability could not be impaired until exposure to 100 mg L arsenite. The thickness of the postsynaptic density (PSD) decreased, whereas the width of the synaptic cleft widened significantly in arsenite exposure groups. Moreover, protein expression of both PSD-95 and SYP decreased significantly in arsenite exposure groups. In conclusion, the results of this study demonstrated that developmental arsenite exposure could depress the expression of synaptic proteins, subsequently cause alteration in synaptic structures, and finally contribute to arsenite-induced deficits in spatial learning and memory ability in mouse offspring.