Increased atmospheric CO2 concentrations lead to decreased pH and carbonate availability in the ocean (Ocean Acidification, OA). Carbon dioxide seeps serve as 'windows into the future' to study the ability of marine invertebrates to acclimatise to OA. We studied benthic foraminifera in sediments from shallow volcanic CO2 seeps in Papua New Guinea. Conditions follow a gradient from present day pH/pCO2 to those expected past 2100. We show that foraminiferal densities and diversity declined steeply with increasing pCO2. Foraminifera were almost absent at sites with pH < 7.9 (>700 μatm pCO2). Symbiont-bearing species did not exhibit reduced vulnerability to extinction at <7.9 pH. Non-calcifying taxa declined less steeply along pCO2 gradients, but were also absent in samples at pH < 7.9. Data suggest the possibility of an OA induced ecological extinction of shallow tropical benthic foraminifera by 2100; similar to extinctions observed in the geological past.