Anthropogenic disturbances like ocean acidification (OA) and nutrient input can undermine the competitive ability of reef-building corals leading to community shifts towards fast-growing organisms. Through a 28-days experiment, we assessed the effects of OA (450 and 950 μatm pCO2) and nitrate enrichment (0, 4, and 8 μmolL-1) on the reef-building coral Stylophora pistillata and soft coral Xenia sp. and the outcome of their competition by direct contact. We measured photosynthetic efficiency (PE) of S. pistillata, growth, zooxanthellae density, and chlorophyll-a concentration of both corals as proxies for their competitive performance. S. pistillata was negatively affected upon competition with Xenia sp. showing reduced PE, zooxanthellae and chlorophyll-a, but abiotic factors either mitigated or aggravated these effects. While acidification impaired the growth of competing reef-building corals, it enhanced the responses of its symbionts. Nitrate input alleviated the acidification impact on the growth of reef-building corals, and increased the zooxanthellae densities and chlorophyll-a contents in competing Xenia sp. Although increasing nitrate benefited the growth of Xenia sp. alone, it reduced their growth when competing with S. pistillata. This experiment demonstrates that OA and nitrate enrichment are interlinked and may play an important role in determining whether future reefs will be dominated by reef-builders.