In this work, poly[1-vinyl-3(2-carboxyethyl) imidazolium betaine] (PVCIB), as a zwitterionic polyelectrolyte, was tethered onto a commercial thin film composite polyamide (TFC PA) membrane. First, polyvinyl imidazole (PVI) was grafted onto the TFC PA membrane surface by free radical graft polymerization method at various grafting times. Afterwards, one of PVI-modified membranes was betainized using 3-bromopropionic acid to obtain PVCIB brushes on the membrane surface. Evaluation of membrane performance through desalination process indicated that despite decrease of salt rejection, water flux increased from 73.4 L/m(2) h in the PA membrane to 91.6 L/m(2) h in the PA-PVCIB membrane. Antimicrobial assessment using Escherichia coli showed that the PVCIB-modified membrane was able to inhibit bacterial growth by about 98.8%. Antifouling and cleaning abilities of membranes were investigated using BSA and lysozyme at various pH values. It was revealed that hydrophilic PVCIB brushes considerably improved protein-resistant property of the TFC PA membrane. However, by considering pH-dependent behavior of PVCIB (zwitterionic at alkaline pH or polyelectrolyte at acidic pH), it was found that hydration repulsion or electrostatic repulsive forces also made a major contribution to the fouling mitigation. Accordingly, the PA-PVCIB membrane exhibited remarkable antifouling ability to resist non-specific protein adsorption at neutral and alkaline pHs, whereas both PA-PVI and PA-PVCIB membranes exhibited high resistance to the positively charged lysozyme adhesion at acidic pH.