Surface tension and micellization behavior of an ionic amphiphilic diblock copolymer, poly(n-butyl acrylate)-b-poly(styrenesulfonate) sodium salt, which shows non-surface active nature in pure water, were investigated in seawater. The block copolymer became surface active in seawater, but the relationship between polymer concentration and surface tension showed a second bending point at a higher concentration in addition to that at the critical micelle concentration (CMC). Polymer micelles were also found in seawater, although the hydrodynamic size was very small. However, larger aggregation numbers of micelles were found in solutions above the second bending point. Experiments using solutions with one, two, or three salt species revealed that cmc was governed by sodium chloride, which is the most abundant salt in seawater, but the micelle structure was sensitive to divalent ions such as Ca2+ and Mg2+, although their amount is very small compared to that of NaCl.