Researchers have investigated the immediate effects of end-of-century climate change scenarios on many marine species, yet it remains unclear whether we can reliably predict how marine species may respond to future conditions because biota may become either more or less resistant over time. Here, we examined the role of pre-exposure to elevated temperature and reduced pH in mitigating the potential negative effects of future ocean conditions on polyps of a dangerous Irukandji jellyfish Alatina alata. We pre-exposed polyps to elevated temperature (28 °C) and reduced pH (7.6), in a full factorial experiment that ran for 14 d. We secondarily exposed original polyps and their daughter polyps to either current (pH 8.0, 25 °C) or future conditions (pH 7.6, 28 °C) for a further 34 d to assess potential phenotypic plastic responses and whether asexual offspring could benefit from parental pre-exposure. Polyp fitness was characterised as asexual reproduction, respiration, feeding, and protein concentrations. Pre-exposure to elevated temperature alone partially mitigated the negative effects of future conditions on polyp fitness, while pre-exposure to reduced pH in isolation completely mitigated the negative effects of future conditions on polyp fitness. Pre-exposure to the dual stressors, however, reduced fitness under future conditions relative to those in the control treatment. Under future conditions, polyps had higher respiration rates regardless of the conditions they were pre-exposed to, suggesting that metabolic rates will be higher under future conditions. Parent and daughter polyps responded similarly to the various treatments tested, demonstrating that parental pre-exposure did not confer any benefit to asexual offspring under future conditions. Importantly, we demonstrate that while pre-exposure to the stressors individually may allow Irukandji polyps to acclimate over short timescales, the stressors are unlikely to occur in isolation in the long term, and thus, warming and acidification in parallel may prevent polyp populations from acclimating to future ocean conditions.