A strategy to create blood-inert surfaces in human whole blood via ion-pair anchoring of zwitterionic copolymer brushesand a systematic study of how well-defined chain lengths and well-controlled surface packing densities of zwitterionic polymers affect blood compatibility are reported. Well-defined diblock copolymers, poly(11-mercaptoundecyl sulfonic acid)-block-poly(sulfobetaine methacrylate) (PSA-b-PSBMA) with varying zwitterionic PSBMA or negatively charged PSA lengths, are synthesized via atom-transfer radical polymerization (ATRP). PSA-b-PSBMA is grafted onto a surface covered with polycation brushes as a mimic polar/hydrophilic biomaterial surface via ion-pair anchoring at a range of copolymer concentrations. Protein adsorption from single-protein solutions, 100% blood serum, and 100% blood plasma onto the surfaces covered with PSA-b-PSBMA brushes is evaluated using a surface plasmon resonance sensor. Copolymer brushes containing a high amount of zwitterionic SBMA units are further challenged with human whole blood. Low protein-fouling surfaces with >90% reduction with respect to uncoated surfaces are achieved with longer PSA blocks and higher concentrations of PSA-b-PSBMA copolymers using the ion-pair anchoring approach. This work provides a platform to achieve the control of various surface parameters and a practical method to create blood-inert surfaces in whole blood by grafting ionic-zwitterionic copolymers to charged biomaterials via charge pairing.