Triple negative breast cancer (TNBC) is the most aggressive breast cancer subtype with the worst prognosis and few effective therapies. Here, we undertook a screen of epigenetic chemical probes to systematically uncover the epigenetic regulators critical for TNBC growth. We identified MS023, an inhibitor of type I protein arginine methyltransferases (PRMTs), as having anti-tumor growth activity in TNBC in vitro and in vivo. Pathway analysis of TNBC cell lines indicates that the activation of interferon responses pre- and post-MS023 treatment is a functional biomarker and determinant of response; and these observations extend to a panel of patient-derived organoids. Inhibition of type I PRMT triggers an interferon response through the antiviral defense pathway with the induction of double-stranded RNA (dsRNA). The observed dsRNA accumulation is derived, at least in part, from inverted-repeat Alus (IR-Alus), many of which are expressed from retained introns induced by MS023, which inhibits arginine methylation of RNA-binding proteins and alters mRNA splicing machinery. Together, our results represent a shift in understanding the anti-tumor mechanism of type I PRMT inhibitors and provide a novel rationale and biomarker approach for the clinical development of type I PRMT inhibitors.