Impaired adult hippocampal neurogenesis is a key pathological mechanism contributing to memory deficits in Alzheimer's disease (AD). Recent studies have shown that elevating magnesium levels promotes neurogenesis by enhancing the neuronal differentiation of adult neural progenitor cells in vitro. Therefore, this in vivo study aims to determine if magnesium-L-threonate (MgT) can ameliorate cognitive deficit of AD mice by attenuating adult hippocampal neurogenesis impairment and to reveal the underlying mechanisms. APPswe/PS1dE9 mice were treated with different doses of MgT and ERK inhibitor PD0325901. The memory ability of each mouse was recorded by Morris Water Maze test. After cognitive test, hippocampus tissues were collected to measure the proportion of BrdU/doublecortin double-labeled cells using the flow cytometry test and assess the expression of doublecortin using PCR and Western blot. Furthermore, the activations of CREB, ERK, P38 and JNK were measured by Western blot to identify the involved mechanisms. The cognitive test confirmed that MgT treatment attenuated the memory impairment of APPswe/PS1dE9 mice. Flow cytometry test showed that Brdu/doublecortin labeled newborn neurons gradually increased following MgT administration. In line with the flow cytometry results, Western blot and PCR confirmed that MgT administration significantly increased doublecortin expression levels. Furthermore, the ratios of p-ERK/ERK and p-CREB/CREB increased with MgT elevation. In addition, these effects of MgT treatment were markedly reversed by PD0325901 supplementation. In conclusion, MgT treatment improved cognitive decline by ameliorating adult hippocampal neurogenesis impairment in this AD model, possibly via ERK/CREB activation.