Polyethylene glycol (PEG) has been widely used as a serum half-life extender of therapeutic proteins. However, due to immune responses and low degradability of PEG, developing serum half-life extender alternatives to PEG is required. Human serum albumin (HSA) has several beneficial features as a serum half-life extender, including a very long serum half-life, good degradability, and low immune responses. In order to further evaluate the efficacy of HSA, we compared the extent of serum half-life extension of a target protein, superfolder green fluorescent protein (sfGFP), upon HSA conjugation with PEG conjugation side-by-side. Combination of site-specific incorporation of p-azido-L-phenylalanine into sfGFP and copper-free click chemistry achieved the site-specific conjugation of a single HSA, 20 kDa PEG, or 30 kDa PEG to sfGFP. These sfGFP conjugates exhibited the fluorescence comparable to or even greater than that of wild-type sfGFP (sfGFP-WT). In mice, HSA-conjugation to sfGFP extended the serum half-life 9.0 times compared to that of unmodified sfGFP, which is comparable to those of PEG-conjugated sfGFPs (7.3 times for 20 kDa PEG and 9.5 times for 30 kDa PEG). These results clearly demonstrated that HSA was as effective as PEG in extending the serum half-life of a target protein. Therefore, with the additional favorable features, HSA is a good serum half-life extender of a (therapeutic) protein as an alternative to PEG.