In this study, we demonstrated that inhibition of Drp1 hyperactivation using P110, a selective peptide inhibitor of Drp1/Fis1 interaction that we developed, reduced pathological mitochondrial fission in ALS using several ALS patient‐derived fibroblasts, motor neurons expressing G93A mutation, and a SOD1G93A mouse model. Specifically, we found that (i) ALS patient‐derived fibroblasts carrying SOD1, FUS1, or TDP‐43 mutations all exhibit a significantly increased Drp1‐recruitment to the mitochondria, increased mitochondrial fragmentation, and subsequent mitochondrial dysfunction; and these were all greatly reduced by P110. (ii) We found that neuronal cells in culture, carrying the SOD1 G93A mutant (a model of ALS in culture), also show increased mitochondrial dysfunction and increased cell death, which was dependent on Drp1 hyperactivation and was inhibited by P110 treatment. (iii) We demonstrated that inhibition Drp1/Fis1 interaction in vivo using P110 was safe and reduced motor/muscle deficit and increased survival in SOD1 G93A mouse model.