Parkinson’s disease is a common progressive neurodegenerative disorder caused by the loss of dopaminergic neurons in the substantia nigra. We investigated whether cell therapy with human mesenchymal stem cells (hMSCs) had a protective effect on progressive dopaminergic neuronal loss in vitro and in vivo. In primary mesencephalic cultures, hMSCs treatment significantly decreased MG‐132‐induced dopaminergic neuronal loss with a significant reduction of caspase‐3 activity. In rats received systemic injection of MG‐132, hMSCs treatment in MG‐132‐treated rats dramatically reduced the decline in the number of tyrosine hydroxylase (TH)‐immunoreactive cells, showing an approximately 50% increase in the survival of TH‐immunoreactive cells in the substantia nigra compared with the MG‐132‐treated group. Additionally, hMSC treatment significantly decreased OX‐6 immunoreactivity and caspase‐3 activity. Histological analysis showed that the number of NuMA‐positive cells was 1.7% of total injected hMSCs and 35.7% of these cells were double‐stained with NuMA and TH. Adhesive‐removal test showed that hMSCs administration in MG‐132‐treated rats had a tendency to decrease in the mean removal time. This study demonstrates that hMSCs treatment had a protective effect on progressive loss of dopaminergic neurons induced by MG‐132 in vitro and in vivo. Complex mechanisms mediated by trophic effects of hMSCs and differentiation of hMSCs into functional TH‐immunoreactive neurons may work in the neuroprotective process.