Myotonic dystrophy type 1 (DM1) is a multisystemic genetic disorder caused by the CTG repeat expansion in the 3′-untranslated region of DMPK gene. Heart dysfunctions occur in ∼80% of DM1 patients and are the second leading cause of DM1-related deaths. Herein, we report that upregulation of a non-muscle splice isoform of RNA-binding protein RBFOX2 in DM1 heart tissue—due to altered splicing factor and microRNA activities—induces cardiac conduction defects in DM1 individuals. Mice engineered to express the non-muscle RBFOX2₄₀ isoform in heart via tetracycline-inducible transgenesis, or CRISPR/Cas9-mediated genome editing, reproduced DM1-related cardiac conduction delay and spontaneous episodes of arrhythmia. Further, by integrating RNA binding with cardiac transcriptome datasets from DM1 patients and mice expressing the non-muscle RBFOX2 isoform, we identified RBFOX2₄₀-driven splicing defects in voltage-gated sodium and potassium channels, which alter their electrophysiological properties. Thus, our results uncover a trans-dominant role for an aberrantly expressed RBFOX2₄₀ isoform in DM1 cardiac pathogenesis.