Summary Plant organ size and shape are major agronomic traits that depend on cell division and expansion, which are tightly regulated by complex gene networks. In several eudicot species belonging to the rosid clade, organ growth is controlled by a repressor complex consisting of PEAPOD (PPD) and KINASE-INDUCIBLE DOMAIN INTERACTING (KIX) proteins. Whether the function of these proteins as regulators of organ size is conserved in asterids as well, which together with the rosids constitute most of the core eudicot species, is still unknown. Here, we demonstrate that KIX8 and KIX9 redundantly regulate organ growth in the asterid model species tomato ( Solanum lycopersicum ). Protein interaction assays in yeast revealed that KIX8 and KIX9 act as molecular bridges between the PPD repressor and TOPLESS co-repressor proteins. We found that CRISPR-Cas9 genome editing of KIX8 and KIX9 led to the production of enlarged, dome-shaped leaves and that these leaves exhibited increased expression of putative PPD target genes. Moreover, the kix8 kix9 mutants carried bigger fruits with increased pericarp thickness. Our results show that KIX8 and KIX9 are conserved regulators of organ size in distinct eudicot species and can thus provide strategies to improve yield in fruit crops.