Phosphoenolpyruvate carboxylase (PEPC) is the second major carbon‐fixing enzyme in photoautotrophic organisms. PEPC is required for the synthesis of amino acids of the glutamate and aspartate family by replenishing the TCA cycle. Furthermore, in cyanobacteria, PEPC, together with malate dehydrogenase and malic enzyme, forms a metabolic shunt for the synthesis of pyruvate from PEP. During this process, CO2 is first fixed and later released again. Due to its central metabolic position, it is crucial to fully understand the regulation of PEPC. Here, we identify PEPC from the cyanobacterium Synechocystis sp. PCC 6803 (PEPC) as a novel interaction partner for the global signal transduction protein PII. In addition to an extensive characterization of PEPC, we demonstrate specific PII–PEPC complex formation and its enzymatic consequences. PEPC activity is tuned by the metabolite‐sensing properties of PII: Whereas in the absence of PII, PEPC is subjected to ATP inhibition, it is activated beyond its basal activity in the presence of PII. Furthermore, PII–PEPC complex formation is inhibited by ADP and PEPC activation by PII‐ATP is mitigated in the presence of 2‐OG, linking PEPC regulation to the cell's global carbon/nitrogen status. Finally, physiological relevance of the in vitro measurements was proven by metabolomic analyses of Synechocystis wild‐type and PII‐deficient cells.