RNA-binding proteins (RBPs) of the STAR family play key roles in mammalian development, yet their contributions to lineage specification remain incompletely understood. Here, using CRISPR-Cas9 knockout models combined with multi-omics approaches, we investigate the functions of two STAR proteins, SAM68 and QKI, in mouse embryonic stem cells (mESCs). Both RBPs support mESC proliferation, self-renewal, and efficient differentiation into cardiomyocytes. Although SAM68 and QKI belong to the same protein family, they control largely distinct regulatory programs during differentiation. We uncover an unexpected role for SAM68 in cardiomyocyte specification through multiple post-transcriptional mechanisms. SAM68 modulates alternative splicing and promotes the biogenesis of a subset of cardiac-enriched circular RNAs, through binding to intronic regions flanking back-splice junctions and potentially through association with NF90/110. In addition, SAM68 binds untranslated regions of key differentiation-related transcripts, including Gata4 mRNA, and functions in ribonucleoprotein complexes to regulate their translation. Together, these findings identify SAM68 as a multifunctional regulator coordinating multiple layers of RNA metabolism-including splicing, circRNA biogenesis, and translation-during cardiomyocyte differentiation and provide insight into how STAR proteins shape post-transcriptional gene regulatory networks during early development.