Manipulative studies have demonstrated that ocean acidification (OA) is a threat to coral reefs, yet no experiments have employed diurnal variations in pCO2 that are ecologically relevant to many shallow reefs. Two experiments were conducted to test the response of coral recruits (less than 6 days old) to diurnally oscillating pCO2; one exposing recruits for 3 days to ambient (440 matm), high (663 matm) and diurnally oscillating pCO2 on a natural phase (420–596 matm), and another exposing recruits for 6 days to ambient (456 matm), high (837 matm) and diurnally oscillating pCO2 on either a natural or a reverse phase (448–845 matm). In experiment I, recruits exposed to natural-phased diurnally oscillating pCO2 grew 6–19% larger than those in ambient or high pCO2. In experiment II, recruits in both high and natural-phased diurnally oscillating pCO2 grew 16 per cent larger than those at ambient pCO2, and this was accompanied by 13–18% higher survivorship; the stimulatory effect on growth of oscillatory pCO2 was diminished by administering high pCO2 during the day (i.e. reverse-phased). These results demonstrate that coral recruits can benefit from ecologically relevant fluctuations in pCO2 and we hypothesize that the mechanism underlying this response is highly pCO2-mediated, night-time storage of dissolved inorganic carbon that fuels daytime calcification.