RNA-binding proteins (RBPs) and microRNAs (miRNAs) are potent post-transcriptional regulators of gene expression. Here, we show that the RBP HuR reduced c-Myc expression by associating with the c-Myc 3′ untranslated region (UTR) next to a miRNA let-7-binding site. Lowering HuR or let-7 levels relieved the translational repression of c-Myc. Unexpectedly, HuR and let-7 repressed c-Myc through an interdependent mechanism, as let-7 required HuR to reduce c-Myc expression and HuR required let-7 to inhibit c-Myc expression. Our findings suggest a regulatory paradigm wherein HuR inhibits c-Myc expression by recruiting let-7-loaded RISC (RNA miRNA-induced silencing complex) to the c-Myc 3′UTR.