Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are autosomal dominant neuromuscular diseases caused by microsatellite expansions and belong to the family of RNA dominant disorders. Availability of cellular models in which the DM mutation is expressed within its natural context is essential to facilitate efforts to identify new therapeutic compounds. Here we generated immortalized DM1 and DM2 human muscle cell lines that display nuclear RNA-aggregates of expanded repeats, a hallmark of myotonic dystrophy. Selected clones of DM1 and DM2 immortalized myoblasts behave as parental primary myoblasts with a reduced fusion capacity of immortalized DM1 myoblasts when compared to control and DM2 cells. Alternative splicing defects were observed in differentiated DM1 but not in DM2 muscle cell lines. Splicing alterations did not result from differentiation delay because similar changes were found in immortalized DM1 transdifferentiated fibroblasts in which the myogenic differentiation has been forced by MyoD overexpression. As a proof-of-concept, we showed that antisense approaches alleviate disease-associated defects and a RNA-seq analysis confirmed that the vast majority of misspliced events in immortalized DM1 muscle cells were affected by antisense treatment, with half of them significantly rescued in treated DM1 cells. In summary, immortalized DM1 muscle cell lines display characteristic disease-associated molecular features such as nuclear RNA-aggregates and splicing defects that can be used as robust readouts for the screening of therapeutic compounds. Therefore, immortalized DM1 and DM2 muscle cell lines represent new models and tools to investigate molecular pathophysiologic mechanisms and evaluate in vitro effects of compounds on RNA toxicity associated with myotonic dystrophy mutations.