Pharmacologic blockade of the myostatin (Mstn)/activin receptor pathway is being pursued as a potential therapy for several muscle wasting disorders. The functional benefits of blocking this pathway are under investigation, in particular given the findings that greater muscle hypertrophy results from Mstn deficiency arising from genetic ablation compared to post‐developmental Mstn blockade. Using high‐resolution MS coupled with SILAC mouse technology, we quantitated the relative proteomic changes in gastrocnemius muscle from Mstn knockout (Mstn−/−) and mice treated for 2‐weeks with REGN1033, an anti‐Mstn antibody. Relative to wild‐type animals, Mstn−/− mice had a two‐fold greater muscle mass and a >1.5‐fold change in expression of 12.0% of 1137 quantified muscle proteins. In contrast, mice treated with REGN1033 had minimal changes in muscle proteome (0.7% of 1510 proteins >1.5‐fold change, similar to biological difference 0.5% of 1310) even though the treatment induced significant 20% muscle mass increase. Functional annotation of the altered proteins in Mstn−/− mice corroborates the mutiple physiological changes including slow‐to‐fast fiber type switch. Thus, the proteome‐wide protein expression differs between Mstn−/− mice and mice subjected to specific Mstn blockade post‐developmentally, providing molecular‐level insights to inform mechanistic hypotheses to explain the observed functional differences.