Proteins like albumin are found in various environmental and living systems, and they have wide applications in various fields. It is known that the functional, conformational and sorption properties of proteins are significantly affected by various surrounding conditions and chemicals. Micro-/nano-plastics are an emerging issue for the environment, living systems and industrial applications, and they can easily leach, sorb and/or desorb chemicals. These processes can change medium characteristics. However, studies on the impact of micro-/nano-plastics on the chemical and biological behaviors of proteins are lacking. Herein, we investigated the interactions between bovine serum albumin and polyethylene terephthalate micro-/nano-plastics by assessing the binding, structural and oxidative characteristics of proteins using UV-VIS, fluorescence and Raman spectroscopies and molecular docking studies. Additionally, the biological impact of non-treated and micro-/nano-plastic-treated proteins was examined by assessing cytotoxicity (mitochondrial activities and membrane integrity) and oxidative stress (antioxidants, reactive oxygen species, catalase, glutathione reductase, and superoxide dismutase) of a human lung epithelial cell (A549) in vitro model. Binding results showed that micro-/nano-plastics had an affinity for proteins and varied according to the exposure concentration and duration. Molecular simulations revealed that micro-/nano-plastics were bound to the active sites of proteins, which caused structural and functional changes. Raman spectral results further confirmed the structural changes in the proteins after the treatments. Moreover, it was observed that the chemical (e.g., zeta potentials, aromatic side chains and folding) and oxidative indicators of proteins were significantly affected. The exposure of lung cells to non-treated and micro-/nano-plastic-treated proteins resulted in different mitochondrial and membrane activities. The oxidative stress indicators revealed that antioxidants, reactive oxygen species and their balance were significantly affected, and the cell viabilities of superoxide dismutase and glutathione reductase were more influenced than those of catalase. The correlation results also indicated that folding, aromatic chain, quenching constant and oxidative potentials of proteins were more effective indicators of the cellular responses of micro-/nano-plastics-treated proteins than zeta potentials. Thus, all the results indicated the side effects of micro-/nano-plastics on proteins owing to their leaching and sorption.