Extracellular vesicles (EVs) secreted by therapeutic stromal cells, such as cardiosphere-derived cells (CDCs), are mediators of disease-modifying bioactivity. CDC-EVs are richly diverse in noncoding RNA (ncRNA), which upon internalization by recipient cells, trigger global changes in gene expression and modify cell behavior. Characterization of such ncRNA creates opportunities for identifying new therapeutic candidates. Already, for example, studies of small Y-RNAs mined from CDC-EVs have revealed unexpected, impactful modulation of gene expression. Here, we tested the effects of orally-delivered TY1, a modified fragment of the human Y-RNA4 gene (plentiful in CDC-EVs), in mdx mice. Relative to control mdx mice, those fed TY1 twice-weekly for 8 weeks had improved heart and skeletal muscle function. Histology revealed decreased interstitial fibrosis in both the heart and tibialis anterior (TA) of mdx mice fed TY1. Turning to myogenesis, TY1 boosted the number of Pax7+ myogenic precursor cells, MyoD+ myoblasts, and neonatal myosin heavy chain myofibers in the TA. In terms of mechanism, TY1 targets macrophages, which underlie its bioactivity. Using a novel macrophage-depletable mdx mouse model, we show macrophages are required for TY1 to be effective. Fluorescent tracing experiments show TY1 is taken up by circulating leukocytes and muscle macrophages, increasing the numbers of M2-like macrophages in the TA of mdx mice. In cell culture, TY1 induces robust transcriptomic changes to bone marrow-derived macrophages, which are expected to modify macrophage biofunction. Together, these data provide compelling evidence demonstrating TY1 has disease-modifying bioactivity and effectively targets macrophages in mdx mice. No FDA-approved therapy has been shown to reverse key disease manifestations in DMD. Thus, TY1 represents a promising new therapeutic candidate for DMD.