RNA-binding proteins (RBPs) determine RNA fate from synthesis to decay. Employing two complementary protocols for covalent UV crosslinking of RBPs to RNA, we describe a systematic, unbiased, and comprehensive approach, termed “interactome capture,” to define the mRNA interactome of proliferating human HeLa cells. We identify 860 proteins that qualify as RBPs by biochemical and statistical criteria, adding more than 300 RBPs to those previously known and shedding light on RBPs in disease, RNA-binding enzymes of intermediary metabolism, RNA-binding kinases, and RNA-binding architectures. Unexpectedly, we find that many proteins of the HeLa mRNA interactome are highly intrinsically disordered and enriched in short repetitive amino acid motifs. Interactome capture is broadly applicable to study mRNA interactome composition and dynamics in varied biological settings. Display omitted ► Interactome capture identifies hundreds of RBPs previously unknown to bind RNA ► Some globular and disordered regions represent unorthodox binding architectures ► Many RNA binders are linked to Mendelian disease ► Metabolic enzymes moonlighting as RBPs Interactome capture identifies the landscape of mRNA-binding proteins in mammalian cells, revealing new roles for metabolic enzymes and a propensity for globular and unstructured regions to associate with messages.