Ultrasmall gold nanoparticles (AuNPs) are an emerging class of nanomaterials exhibiting distinctive physicochemical, molecular, and in vivo properties. Recently, we showed that ultrasmall AuNPs encompassing a zwitterionic glutathione monoethyl ester surface coating (AuGSH(zwt)) were highly resistant to aggregation and serum protein interactions. Herein, we performed a new set of biointeraction studies to gain a more fundamental understanding into the behavior of both pristine and peptide-functionalized AuGSH(zwt) in complex media. Using the model Strep-tag peptide (WSHPQFEK) as an integrated functional group, we established that AuGSH(zwt) could be conjugated with increasing numbers of Strep-tags by simple ligand exchange, which provides a generic approach for AuGSH(zwt) functionalization. It was found that the strep-tagged AuGSH(zwt) particles were highly resistant to nonspecific protein interactions and retained their targeting capability in biological fluid, displaying efficient binding to Streptactin receptors in nearly undiluted serum. However, AuGSH(zwt) functionalized with multiple Strep-tags displayed somewhat lower resistance to protein interactions and lower levels of binding to Streptactin than monofunctionalized AuGSH(zwt) under given conditions. These results underscore the need for optimizing ligand density onto the surface of ultrasmall AuNPs for improved performance. Collectively, our findings support ultrasmall AuGSH(zwt) as an attractive platform for engineering functional, protein-mimetic nanostructures capable of specific protein recognition within the complex biological milieu.