Protein adsorption influences greatly the performance of materials used in biotechnology and biomedicine. The binding of fibrinogen (Fg) to nanoparticles (NPs) can result in protein unfolding and exposure of cryptic epitopes that subsequently interact with cell surface receptors. The response and its degree are dependent on the size, charge, and concentration of the NPs. In this study the binding kinetics of human Fg to negatively charged 11-mercaptoundecanoic acid-functionalized gold nanoparticles (AuNPs–COOH) ranging from 5.6 to 64.5 nm were examined. The larger NPs bound Fg with a larger number of proteins per square unit and a higher dissociation rate (K_d′), but with decreased affinity. By contrast, the 5.6 nm AuNPs–COOH behaved in a cooperative manner for Fg adsorption. In the presence of excess Fg, only the 64.5 nm AuNPs–COOH showed severe aggregation, whose degree was alleviated in a dilute Fg solution. The Fg is adsorbed through a side-on configuration and both side-on and end-on configurations on the smaller (5.6 and 14.2 nm) and 31.5 nm AuNPs–COOH, respectively. It also retains the native conformation. By contrast, on the 64.5 nm AuNPs–COOH the Fg adopts the end-on configuration and loses most of the secondary structure.