The hemolymph acid–base status of Akoya pearl oyster Pinctada fucata martensii exposed to air at 20°C was investigated. Air-exposed Akoya pearl oyster showed a decrease in hemolymph pH from 7.568 to 6.825 after 24 h. The hemolymph total CO₂ concentration increased from 2.25 mM/L to 4.50 mM/L during 24 h of air exposure. The hemolymph CO₂ partial pressure (Pco₂) was calculated by rearranging the Henderson–Hasselbalch equation. The hemolymph Pco₂ increased from 1.0 torr to 14.8 torr, and bicarbonate ion concentration increased from 2.21 mM/L to 3.91 mM/L during 24 h of air exposure. The hemolymph calcium ion concentration ([Ca²⁺]) increased from 9.4 mM/L to 12.8 mM/L. These results indicated that Akoya pearl oysters showed hemolymph acidosis with partial metabolic compensation by mobilization of bicarbonate from the shell valve during prolonged air exposure. Immersion in seawater for between 4 h and 24 h decreased the effect of air exposure on hemolymph acid–base status, except for [Ca²⁺]. The hemolymph [Ca²⁺] of immersed Akoya pearl oysters was slightly higher than the initial level upon air exposure although hemolymph [Ca²⁺] had already decreased after immersion in seawater for 4 h and 24 h. The hemolymph acid–base balance of immersed Akoya pearl oysters recovered to the initial level after 4–24 h, even if the animals were exposed to the air for a prolonged time (24 h).