Superabsorbent polymer gels were synthesized by terpolymerization of three kinds of tri-n-alkyl-4-vinylbenzyl phosphonium chloride (TRVB) with alkyl chains of different lengths, with acrylamide (AAm), and with N,N'-methylenebisacrylamide (MBAAm). The water-absorption ability and antibacterial activity of the gels against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coil) were investigated. The water content of TRVB-AAm-MBAAm terpolymers increased with increasing phosphonium groups in the terpolymers, while the water content decreased with increasing chain length of alkyl groups in the phosphonium groups as well as an increasing degree of crosslinking in the terpolymers. The water content of the terpolymers was depressed by the addition of NaCl. The degree of effect of NaCl addition became higher as the chain length of alkyl groups in the phosphonium groups of the terpolymers became longer. The tri-n-octyl-4-vinylbenzyl phosphonium chlorides (TOVB)-AAm-MBAAm terpolymers exhibited high antibacterial activity against S. aureus and E. coli in deionized water. The antibacterial activity decreased in 0.9 wt % NaCl solution. The antibacterial activity of TOVB-AAm-MBAAm terpolymers with almost the same phosphonium content increased with the increasing swelling ratio of the terpolymers.