Poor pharmacokinetic profiles are often the underlying reason for the failure of novel protein drugs to reach clinical translation. Current passive half-life improvement methods focus on increasing the apparent hydrodynamic radius of the drug. We sought to develop an active method to increase the circulation half-life of proteins by binding to erythrocytes in blood. Screening a naive phage-displayed peptide library against whole mouse erythrocytes yielded a 12 amino acid peptide (ERY1) that binds the erythrocyte surface with high specificity. ERY1-displaying phage bind mouse and rat erythrocytes 95-fold higher than wild-type phage and exhibit negligible binding to mouse leukocytes, as determined by flow cytometry. Affinity experiments with soluble peptide revealed the extracellular domain of glycophorin-A as the membrane protein ligand. When expressed as an N-terminal fusion to maltose-binding protein and administered intravenously, the erythrocyte-binding variant exhibits a 3.2- to 6.3-fold increase in circulation half-life, 2.15-fold decrease in clearance, and 1.67-fold increase in bioavailability as compared to the wild-type protein. The peptide fails to induce ERY1-reactive immunoglobulin production, furthering the potential of the concept in therapeutic design, although this sequence does not bind human erythrocytes. We conclude that engineering erythrocyte affinity into proteins effectively increases their circulation half-life, thereby offering a solution to improve pharmacokinetic profiles of the numerous therapeutic protein drugs in clinical development.