Polyethersulfone (PES) is one of the most used synthetic polymers for the production of hemodialysis membranes, due to its appropriate features, such as biocompatibility, high permeability for low-molecular-weight proteins, high endotoxin retention ability, and resistance to sterilization processes. However, there is room for improvement regarding their anticoagulant properties when coming into contact with blood. In the present study, commercial PES membranes were plasma-treated and then chemically modified with crown ether, an organic compound that could interfere with the coagulation cascade by complexating Ca in the blood. The physico-chemical and morphological characteristics of the membranes were determined by FT-IR, XPS, TGA, SEM, and CT analyses, while their efficiency in retaining calcium ions was evaluated via ICP-MS. The results revealed that plasma treatment with a mixture of argon and ammonia was the most effective in generating nitrogen-containing surface functional groups and that these moieties can be successfully used for the covalent functionalization of the membranes. Also, the Ca retention ability of the PES membranes was improved by up to 30% after chemical modification with 4'-aminobenzo-15-crown-5 ether.