Genetic fusion of a therapeutic protein to albumin can improve its stability and pharmacokinetics, but it usually leads to considerably reduced bioactivity and poor tumor penetration due to increased steric hindrance, resulting in limited antitumor efficacy. Herein we report head-to-tail macrocyclization of albumin-binding domain fused interferon alpha (IFN-ABD) to form a cyclic fusion protein (c-IFN-ABD) with well-retained albumin-binding affinity. Notably, c-IFN-ABD showed not only greater thermal and enzymatic stability and thus antiproliferative activity than IFN-ABD and IFN due to the macrocyclization, but also exhibited considerably better pharmacokinetics than IFN and cyclic IFN owing to the albumin-binding affinity. More importantly, c-IFN-ABD showed deeper tumor penetration, greater tumor retention, and thus higher antitumor efficiency than all the controls without significant systemic side effects in mice bearing melanoma. These results implicate that head-to-tail macrocyclization of ABD fused therapeutic proteins is an enabling strategy for the design of highly potent protein therapeutics for tumor therapy.Copyright © 2020 Elsevier Ltd. All rights reserved.