Noncompaction cardiomyopathy is a common congenital cardiac disorder associated with abnormal ventricular cardiomyocyte trabeculation and impaired pump function. The genetic basis and underlying mechanisms of this disorder remain elusive. We show that genetic deletion of RNA binding protein with multiple splicing (Rbpms), an uncharacterized RNA binding and splicing factor, causes perinatal lethality in mice due to congenital cardiovascular defects, including noncompaction cardiomyopathy, ventricular septal defects, and patent ductus arteriosus. Loss of Rbpms causes premature onset of cardiomyocyte binucleation and cell cycle arrest during development, thus disturbing embryonic heart growth. Human iPSC-derived cardiomyocytes with RBPMS gene deletion have a similar blockade to cytokinesis, and Rbpms expression is decreased in human patients with congenital cardiomyopathy, implicating an essential role of Rbpms in human heart development. Paired-end RNA sequencing analysis revealed that RBPMS plays a role in RNA splicing and influences RNAs involved in cytoskeletal signaling pathways. We found that RBPMS mediates isoform switching of the heart-specific LIM domain protein Pdlim5. Loss of Rbpms leads to abnormal accumulation of Pdlim5-short isoforms, disrupting cardiomyocyte cytokinesis. Our findings connect premature cardiomyocyte binucleation to noncompaction cardiomyopathy and highlight the role of Rbpms in this process.