We investigated the occurrence and isotope signature of Ammonia “beccarii” forma 1 on a bridge pier of the Ohashi River, southwest Japan, to elucidate the characteristics of benthic foraminifera on hard substrates in brackish water. Ammonia “beccarii” forma 1 occurred within the attached macrobenthos community on the bridge pier as well as on the bottom sediments of the nearby river and lake floor. Test morphology of bridge-pier specimens is similar to that of the surface-sediment specimens. A culturing study examining substrate preferences revealed that bridge-pier individuals of A. “beccarii” forma 1 most commonly attached to the body of hydrozoans, although many individuals established themselves in the sediments. Our observations suggest that the bridge-pier population is derived from the surface-sediment community, in particular, that from the river floor. Stable oxygen and carbon isotope ratios of specimens of A. “beccarii” forma 1 from the bridge pier have a smaller range than those in surface sediment from the Ohashi River and Lake Nakaumi. Stable isotope compositions of tests of A. “beccarii” forma 1 on and around the bridge pier have a weakly negative relationship between oxygen and carbon isotope ratios. This may reflect the influence of both saline bottom water and pore water within river sediments. The stable isotope composition of foraminiferal specimens from the bridge pier has a strong potential for environmental monitoring of water-column chemistry in local brackish environments.