Calcifying organisms are considered as threatened by ocean acidification, because of their calcium carbonate skeleton. This study investigated if a calcareous sponge could synthesize its skeleton (i.e. spicules) under ocean-acidification conditions. Sponge cell aggregates that have the potential to develop into a functional sponge, called primmorphs, were submitted to a 5-day experiment, with two treatments: control (pH 8.1) and acidified conditions (pH 7.6). Primmorphs of the calcareous sponge Paraleucilla magna were able to synthesize a skeleton, even under low pH, and to develop into functional sponges. The spicules had the same shape in both conditions, although the spicules synthesized in low pH were slightly thinner than those in the control. These results suggest that P. magna may be able to survive near-future ocean-acidification conditions.