Significance Photosynthetic organisms are responsible for the incorporation of inorganic carbon in the biosphere through a fundamental process known as carbon fixation. This reaction allows the reduction of inorganic carbon, mostly atmospheric CO 2 , to organic compounds such as carbohydrates and amino acids. Despite the biological relevance of carbon fixation in nature, how photosynthetic cells sense carbon availability remains poorly understood. Using the model microalga Chlamydomonas reinhardtii , we found that the photosynthetic assimilation of CO 2 regulates the activity of the target of rapamycin (TOR) kinase, a master regulator of cell growth and nutrient sensor widely conserved in all eukaryotes. Our study revealed that inorganic carbon fixation and photosynthesis regulate TOR activity, likely through the synthesis of central amino acids in carbon metabolism.