Crustose coralline algae (CCA) play a crucial role in the building of reefs in the photic zones of nearshore ecosystems globally and are highly susceptible to ocean acidification. Yet the extent to which CCA can gain tolerance to ocean acidification over multiple generations of exposure is unknown. We show that while calcification of juvenile CCA is initially highly sensitive to ocean acidification, after 6 generations of exposure the effects of ocean acidification disappears. A reciprocal transplant experiment conducted on the 7^th generation where half of all replicates were interchanged across treatments confirmed that they had acquired tolerance to low pH and not simply to laboratory conditions. Neither exposure to greater pH variability, nor chemical conditions within the micro-scale calcifying fluid internally, appeared to play a role in fostering this capacity. Our results demonstrate that reef-accreting taxa can gain tolerance to ocean acidification over multiple generations of exposure, suggesting that some of these cosmopolitan species could maintain their critical ecological role in reef-formation.