Background and Purpose Chemical conjugation of therapeutic proteins with polyethylene glycol (PEG) is an established strategy to extend their biological half-life (t1/2) to a clinically useful range. We developed a novel uncharged and unstructured recombinant polypeptide composed of five amino acids (P, S, T, A and G), named PsTag, as another approach to extend the t1/2 of human FGF21, with increased hydrodynamic radius. Experimental Approach Human FGF21 was fused with PsTag polymers of differing lengths (200 - 600 residues). Three fusion proteins and native FGF21 were produced in Escherichia coli. The biophysical characteristics, metabolic stability, immunogenicity and pharmacokinetics in were assessed in first. In lean and diet-induced obese (DIO) mice, effects on body weight, oral glucose tolerance tests and levels of relevant hormones and metabolites were studied. Key Results Fusion proteins were solubly expressed in E. coli and prolonged the t1/2 from 0.34h up to 12.9 h in mice. Fusion proteins were also biodegradable, thus avoiding vacuole formation, while lacking immunogenicity in mice. In DIO mice, administration of PsTag fused to FGF21 reduced body weight, blood glucose and lipids levels and reversed hepatic steatosis. Conclusions and Implications The novel recombinant polypeptide, PsTag, should be useful in the development of biological drugs with properties comparable to those achievable by PEGylation, but with potentially less side effects. In mice, fusion of FGF21 to PsTag prolonged and potentiated pharmacological effects of native FGF21, and may offer greater therapeutic effects in treatment of obesity.