C4 and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C4 nor CAM have been recorded for the Basellaceae, a small family in the CAM-rich sub-order Portulacineae. 24 h gas exchange and day–night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet–dry–wet cycles. While net CO2 uptake was restricted to the light period in well-watered plants, net CO2 fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C3 photosynthesis upon rewatering. The reversible induction of nocturnal net CO2 uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.