There is increasing evidence that toxicity of mutant superoxide dismutase-1 (SOD1) in amyotrophic lateral sclerosis (ALS) is linked to its propensity to misfold and to aggregate. Immunotargeting of differently folded states of SOD1 has provided therapeutic benefit in mutant SOD1 transgenic mice. The specific region(s) of the SOD1 protein to which these immunization approaches target are, however, unknown. In contrast, we have previously shown, using a specific antibody [SOD1 exposed dimer interface (SEDI) antibody], that the dimer interface of SOD1 is abnormally exposed both in mutant SOD1 transgenic mice and in familial ALS cases associated with mutations in the SOD1 gene (fALS1). Here, we show the beneficial effects of an active immunization strategy using the SEDI antigenic peptide displayed on a branched peptide dendrimer to target monomer/misfolded in SOD1^G37R and SOD1^G93A mutant SOD1 transgenic mice. Immunization delayed disease onset and extended disease duration, with survival times increased by an average of 40 d in SOD1^G37R mice. Importantly, this immunization strategy favored a Th2 immune response, thereby precluding deleterious neuroinflammatory effects. Furthermore, the beneficial effects of immunization correlated with a reduction in accumulation of both monomer/misfolded and oligomeric SOD1 species in the spinal cord, the intended targets of the immunization strategy. Our results support that SOD1 misfolding/aggregation plays a central role in SOD1-linked ALS pathogenesis and identifies monomeric/misfolded SOD1 as a therapeutic target for SOD1-related ALS.