ATP synthases in chloroplasts (cpATPase) and mitochondria (mtATPase) are responsible for ATP production during photosynthesis and oxidative phosphorylation, respectively. Both enzymes consist of two multisubunit complexes, the membrane-bound coupling factor O and the soluble coupling factor 1. During cpATPase biosynthesis, several accessory factors facilitate subunit production and orchestrate complex assembly. Here, we describe a new auxiliary protein in Arabidopsis thaliana, which is required for cpATPase accumulation. AtCGLD11 (CONSERVED IN THE GREEN LINEAGE AND DIATOMS 11) is a protein without any known functional domain and shows dual localization to chloroplasts and mitochondria. Loss of AtCGLD11 function results in reduced levels of cpATPase and impaired photosynthetic performance with lower rates of ATP synthesis. In yeast two-hybrid experiments, AtCGLD11 interacts with the β subunits of the cpATPase and mtATPase. Our results suggest that AtCGLD11 functions in F₁ assembly during cpATPase biogenesis, while its role in mtATPase biosynthesis may not, or not yet, be essential.