In amyotrophic lateral sclerosis (ALS) caused by SOD1 gene mutations, both cell-autonomous and noncell-autonomous mechanisms lead to the selective degeneration of motoneurons (MN). Here, we evaluate the therapeutic potential of gene therapy targeting mutated SOD1 in mature astrocytes using mice expressing the mutated SOD1(G93A) protein. An AAV-gfaABC(1)D vector encoding an artificial microRNA is used to deliver RNA interference against mutated SOD1 selectively in astrocytes. The treatment leads to the progressive rescue of neuromuscular junction occupancy, to the recovery of the compound muscle action potential in the gastrocnemius muscle, and significantly improves neuromuscular function. In the spinal cord, gene therapy targeting astrocytes protects a small pool of the most vulnerable fast-fatigable MN until disease end stage. In the gastrocnemius muscle of the treated SOD1(G93A) mice, the fast-twitch type IIB muscle fibers are preserved from atrophy. Axon collateral sprouting is observed together with muscle fiber type grouping indicative of denervation/reinnervation events. The transcriptome profiling of spinal cord MN shows changes in the expression levels of factors regulating the dynamics of microtubules. Gene therapy delivering RNA interference against mutated SOD1 in astrocytes protects fast-fatigable motor units and thereby improves neuromuscular function in ALS mice.