Polyethersulfone (PES) is a nondegradable, biocompatible, synthetic polymer that is commonly utilized as a membrane material for applications such as hemodialysis, ultrafiltration and bioreactor technology. Various studies have shown surface modification to be a valuable tool in the development of nondegradable materials which promote cell adhesion. Cells of interest include adipose-derived stem cells (ASCs). ASCs are multipotent mesenchymal stem cells that are useful for various regenerative medicine applications. In this study, we hypothesized that PES surfaces modified with a peptide sequence based from fibronectin, such as Arg-Gly-Asp (RGD), Arg-Gly-Asp-Ser and Gly-Arg-Gly-Asp-Ser, would increase ASC adhesion compared to unmodified PES surfaces. The synthetic peptides were covalently bonded to amine-modified PES surfaces using 1-ethyl-3-(dimethylaminopropyl) carbodiimide. The surfaces were characterized using a ninhydrin assay and contact angle measurements. The ninhydrin assay confirmed the presence of amine groups on the surface of peptide-treated PES disks. Advancing water contact angles were analyzed to detect changes in the hydrophilicity of the polymer surfaces, and results indicated our PES membranes had excellent hydrophilicity. The attachment and proliferation of human ASCs was assessed and RGD-treated surfaces resulted in a higher number of attached ASCs after 6 and 48 h, as compared to unmodified PES surfaces. Additionally, varying concentrations of the RGD peptide sequence concentration were examined. These results indicate that PES membranes modified with the RGD peptide sequence can be utilized for enhanced ASC attachment in biomedical applications.