The oceans have absorbed more than 40% of the carbon dioxide (CO₂) generated by anthropogenic activities, causing a decrease in the average pH of 0.1 units in seawater since preindustrial times. This phenomenon has been called “ocean acidification.” This change poses serious threats to the cultivation of oysters and especially to larval and spat production, activities carried out in coastal and estuarine areas, where pH levels are currently below the IPCC scenario for the year 2100 of pH = 7.8. The goal of the present work was to experimentally evaluate the effect of simulated acidification (pH 7.39 ± 0.04) on the culture in a short-term trial of Kumamoto oyster larvae Crassostrea sikamea taking the current ocean pH conditions (pH 8.116 ± 0.023) as a reference. The evaluation was carried out in an experimental system with continuous water flow and pH manipulation by CO₂ bubbling. Veliger larvae (6-day-old postspawn) were cultured at a density of six larvae mL^–1 and fed with a monoalgal diet based on Isochrysis galbana at 30,000 cells mL^–1. Mortality (%) and growth (shell length in µm) were evaluated, and damage to larval morphology (determined using scanning electron microscopy) and Ca²⁺ contents in the shells (%) were quantified by X-ray fluorescence. The results show a high sensitivity of C. sikamea veliger larvae to low pH levels with negative impacts on growth and survival, decreases in the Ca²⁺ concentrations of the shells, and the presence of morphological anomalies during the prodissoconch I stage, which were observed after the first 24 h of cultivation in experimental conditions and became progressively more evident, especially by the sixth day of culture. Acute exposure to a low pH and a saturation of aragonite (Ωar) <1 caused poor calcification in C. sikamea larvae, causing negative effects on larvae, such as shell lesions during development, smaller larvae, and higher mortality and relation to a control pH.