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While there is an ever-expanding list of impacts on coral reefs as a result of ocean warming and acidification, there is little information on how these global changes influence coral-algal competition. The present study assessed the impact of business-as-usual ocean warming and acidification conditions on the survivorship, calcification, photosynthesis and respiration of the coral-algal interaction between the macroalga Halimeda heteromorpha and the coral Acropora intermedia over 8 weeks in two seasons. The physiological responses of A. intermedia and H. heteromorpha were highly dependent on season, with both organisms demonstrating optimal rates of calcification and photosynthesis under present-day conditions in summer. Contact with H. heteromorpha did not influence A. intermedia survivorship, however did reduce long-term calcification rates. Photosynthetic rates of A. intermedia were influenced by algal contact temporally in opposing directions, with rates reduced in winter and increased in summer. Enhanced photosynthetic rates as a result of algal contact were not enough to offset the combined effects of ocean warming and acidification, which regardless of coral-algal contact, reduced survivorship, calcification and photosynthesis of A. intermedia and the calcification rates of H. heteromorpha. These findings provide experimental support for the idea that the effects of coral-algal competition are temporally variable, and help improve our understanding of how future ocean warming and acidification may alter the dynamics of coral-algal interactions.