To assess how ocean acidification and temperature interactively affect metabolic performance we conducted a mesocosm experiment to compare between two pH levels (ambient ~7.9; reduced ~7.7) across a gradient of eight ecologically relevant temperatures (12°C to 26°C, in 2°C increments). Respiration rates (O₂ consumption), a proxy for energetic expenditure through ATP production were measured after four weeks to generate thermal performance curves. Metabolic responses were then modeled using the Sharpe–Schoolfield equation, which incorporates enzyme kinetics to estimate key physiological parameters, including thermal optima and activation energy, under both ambient and acidified conditions. Results revealed a consistent pattern: sea snails exhibited significantly higher respiration rates near their thermal optimum (~22–24°C) under acidified conditions compared to ambient treatments, indicating that ocean acidification imposes additional metabolic costs even at temperatures typically associated with peak performance. This increase in oxygen demand likely reflects elevated energetic requirements for maintaining acid–base regulation and biomineralization under acidified seawater, where reduced carbonate ion availability (CO₃²⁻) constrains the chemical conditions necessary for shell formation. Together, these findings demonstrate that ocean acidification reshapes thermal performance by elevating metabolic demand, thereby constraining energy budgets and increasing the costs of maintaining essential physiological processes in a rapidly warming ocean.