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Simple Summary: The ocean absorbs large amounts of CO2 emitted by human activities, which leads to a decrease in seawater pH, and has the potential to cause damage to calcareous marine organisms. Planktic foraminifera are some of the most important calcareous marine organisms in the ocean, although the biological response of planktic foraminifera to the decline in seawater pH is still unknown. In this study, the biological response of planktic foraminifera to declining seawater pH was studied through a series of on-board experiments. The experimental results showed that the decrease in seawater pH adversely affected the biological processes of planktic foraminifera, resulting in weaker predation, slower growth, lighter shells, and more deformities. In addition, for the first time, we report that microalgae that live with planktic foraminifera were also killed under low-pH conditions. Several indices were established to quantify the relationships between the biological parameters of planktic foraminifera and seawater pH, which could be used to reconstruct the paleoceanographic seawater pH. This study provides experimental data to quantify the biological response of calcareous plankton to a decline in seawater pH. Understanding the way in which a decline in ocean pH can affect calcareous organisms could enhance our ability to predict the impacts of the potential decline in seawater pH on marine ecosystems, and could help to reconstruct the paleoceanographic events over a geological time scale. Planktic foraminifera are among the most important biological proxies for these studies; however, the existing research on planktic foraminifera is almost exclusively based on their geochemical indices, without the inclusion of information on their biological development. Through a series of on-board experiments in the western tropical Pacific (134°33′54″ E, 12°32′47″ N), the present study showed that the symbiont-bearing calcifier Trilobatus sacculifer—a planktic foraminifer—responded rapidly to a decline in seawater pH, including losing symbionts, bleaching, etc. Several indices were established to quantify the relationships between these biological parameters and seawater pH, which could be used to reconstruct the paleoceanographic seawater pH. We further postulated that the loss of symbionts in planktic foraminifera acts as an adaptive response to the stress of low pH. Our results indicate that an ongoing decline in seawater pH may hinder the growth and calcification of planktic foraminifera by altering their biological processes. A reduction in carbonate deposition and predation could have profound effects on the carbon cycle and energy flow in the marine food web. [ABSTRACT FROM AUTHOR]