Outbreaks of crown-of-thorns starfish Acanthaster spp. (COTS) are among the most severe local threats to Indo-Pacific coral reefs. Despite intensive research, the factors triggering outbreaks remain unclear, though could involve enhanced COTS larval fitness due to ocean warming and acidification. Nevertheless, the effect of these combined stressors has never been tested on larval development and survivorship after parental exposure. We investigated the effects of ocean warming (+2 °C above ambient) and/or acidification (pH 7.75) on early COTS life-history stages of offspring after exposing the parental generation for 20 weeks to the same treatments. We hypothesized that prior adult exposure would modulate the effects measured in previous studies that omitted this phase, providing a more realistic scenario. Our results showed detrimental effects of elevated temperature towards lower gastrulation success and smaller advanced bipinnaria. Both elevated temperature and lower pH produced developmental delay from early to advanced bipinnaria, eventually translating into retarded achievement of mid-late brachiolaria. On average, larvae were significantly bigger in low pH treatments, independent of stages. We suggest a link between developmental delay and larger larvae due to acidification, where larvae could be blocked at a developmental stage but continue growing. Finally, we found that larval mortality was not impacted by treatments, potentially due to prior adult exposure. If adult COTS were able to acclimatize their reproductive physiology in 15 weeks to produce larvae withstanding warming and/or acidification, slow climatic changes might not affect survival at this life stage. However, the developmental delays displayed might elongate their fragile pelagic phase, potentially decreasing their chances to reach recruitment. We specified the natural spawning peak in New Caledonia, and show caution in directly linking high fertilisation rates with high larval success. Our study reinforces the need to include parental exposure when investigating climate change effects on echinoderm larvae, as punctual stress over single-life stages may produce misleading results.