Phenotypic plasticity may be critical to the maintenance of viable populations under future environmental change. Here we examined the role of behavioural avoidance of sub-optimal conditions in enabling the intertidal gastropod, Bembicium auratum, to persist in mangrove forests affected by the low pH runoff from acid sulphate soils (ASS). Behaviourally, the gastropod may be able to avoid periods of particularly high acidity by using pneumatophores and/or mangrove trunks to vertically migrate above the water line or by retreating into its shell. We hypothesised that (1) B. auratum would display greater and more rapid vertical migration out of acidified than reference estuarine waters, and (2) responses would be more pronounced in gastropods collected from acidified than reference sites. Gastropods from acidified sites showed significantly higher activity in and more rapid migration out of acidified waters of pH 6.2–7.0, than reference waters or waters of pH < 5.0. Gastropods from reference locations showed a significantly weaker response to acidified water than those from acidified waters, and which did not significantly differ from their response to reference water. At extremely low pHs, <5.0, a higher proportion of both acidified and reference gastropods retreated into their shell than at higher pHs. Both the migration of gastropods out of acidified waters and retraction into their shells serves to reduce exposure time to acidified waters and may reduce the impact of this stressor on their populations. The stronger response to acidification of gastropods from populations previously exposed to this stressor suggests that the response may be learned, inherited or induced over multiple exposures. Our study adds to growing evidence that estuarine organisms may exhibit considerable physiological and behaviour adaptive capacity to acidification. The potential for such adaptive capacity should be incorporated into studies seeking to forecast impacts to marine organisms of environmental change.