As the ocean warms, the thermal tolerance of marine invertebrates is key to determining their distributional change, where simultaneous changes in ocean pH may impact the thermal range of optimal development. We compared developmental thermal tolerance of multiple male-female crosses from a low pH-adapted sea urchin (Arbacia lixula) population from the CO2 vents of Ischia (Italy) (mean pHT of 7.69) and a nearby population living at ambient pH. The percentage of normally developing gastrulae and two-armed larvae were determined across 10 temperatures representing present and future temperature conditions (16-34 ˚C). Vent-adapted sea urchins showed a greater percentage of normal development at 24 hours, with a larger optimal developmental temperature range than control sea urchins (12.3 ˚C vs 5.4 ˚C range, respectively). At 48 hours, upper lethal temperatures for 50% survival with respect to ambient temperatures were similar between control (+6.8 ˚C) and vent (+6.2 ˚C) populations. Thus, adaptation to low pH did not impact the broad thermal tolerance of development of A. lixula. With A. lixula’s barrens forming abilities, its wide thermotolerance and its ability to adapt to low pH, this species will continue to be ecologically important in Mediterranean kelp forest ecosystems in a future ocean.