Organ-selective delivery of messenger RNA (mRNA) is critical for fulfilling the therapeutic potential of mRNA-based gene and protein replacement technologies. Despite clinical advances in the hepatic delivery of mRNA using lipid nanoparticles (LNPs), current strategies for extrahepatic-organ-selective mRNA delivery still have limitations. Here we report a peptide-encoded organ-selective targeting (POST) method for the delivery of mRNA to extrahepatic organs after systemic administration, which is based on the modular tuning of LNPs through surface engineering with specific amino acid sequences (POST codes). Molecular dynamics simulations and in vitro and in vivo testing show that the organ-selective targeting of POST results from the specific protein corona of the peptide-decorated LNPs, which is established from the mechanical optimization of the binding affinities between peptides with a particular sequence and plasma proteins. This approach can be used for the organ-selective delivery of different ribonucleic acids and multiple gene editing machinery. Overall, the POST platform creates a modular repertoire for LNP surface engineering for directing organ tropism, broadening the scope and versatility of organ-selective delivery.