Pilea peperomioides Diels (Urticaceae) is a semi-succulent herbaceous species native to south-western China that has become popular in cultivation as an ornamental plant. To investigate whether this species possesses the capacity for CAM photosynthesis, measurements were made of CO₂ gas exchange and titratable acidity in plants under both well-watered and water-deficit conditions. Plants were found to assimilate CO₂ almost exclusively in the light via C₃ photosynthesis. However, distinct transient reductions in the rate of net nocturnal CO₂ release were consistently observed during the course of the dark period, and under water-deficit conditions one plant exhibited a brief period of net nocturnal CO₂ uptake, providing unequivocal evidence of CAM activity. Furthermore, nocturnal increases in titratable acidity in both leaf laminas and petioles were observed in all plants exposed to wet–dry–wet cycles. This is the first report of CAM in the family Urticaceae. The results are discussed in relation to the phylogenetic position of Pilea and the partially shaded montane habitats in which this species is typically found. An updated list of all plant families currently known to contain species with CAM is presented.