Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a neuroprotective factor produced in response to endoplasmic reticulum (ER) stress induced by various stressors, but its involvement in the radioresistance of tumor cells is unknown. Here, we found that MANF is released after gamma-irradiation (2 Gy and 4 Gy) of B16 melanoma cells, and its release was suppressed by 4-phenylbutyric acid, an ER stress inhibitor. MANF was not released after low-dose (1 Gy) gamma-irradiation, but pretreatment of 1 Gy-irradiated cells with recombinant MANF enhanced the cellular DNA damage response and attenuated reproductive cell death. In MANF-knockdown cells, the DNA damage response and p53 activation by gamma-irradiation (2 Gy) were suppressed, and reproductive cell death was increased. MANF also activated the ERK signaling pathway. Our findings raise the possibility that MANF could be a new target for overcoming radioresistance. (C) 2020 Elsevier Inc. All rights reserved.