The increase of CO ₂ emissions due to human activity is one of the preeminent reasons for the present climate crisis. In addition, considering the increasing demand for renewable resources, the upcycling of CO ₂ as a feedstock gains an extensive importance to establish CO ₂ -neutral or CO ₂ -negative industrial processes independent of agricultural resources. Here we assess whether synthetic autotrophic Komagataella phaffii ( Pichia pastoris ) can be used as a platform for value-added chemicals using CO ₂ as a feedstock by integrating the heterologous genes for lactic and itaconic acid synthesis. ¹³ C labeling experiments proved that the resulting strains are able to produce organic acids via the assimilation of CO ₂ as a sole carbon source. Further engineering attempts to prevent the lactic acid consumption increased the titers to 600 mg L ⁻¹ , while balancing the expression of key genes and modifying screening conditions led to 2 g L ⁻¹ itaconic acid. Bioreactor cultivations suggest that a fine-tuning on CO ₂ uptake and oxygen demand of the cells is essential to reach a higher productivity. We believe that through further metabolic and process engineering, the resulting engineered strain can become a promising host for the production of value-added bulk chemicals by microbial assimilation of CO ₂ , to support sustainability of industrial bioprocesses.