In tomato, sugar content is highly correlated with consumer preferences, with most consumers preferring sweeter fruit1–4. However, the sugar content of commercial varieties is generally low, as it is inversely correlated with fruit size, and growers prioritize yield over flavour quality5–7. Here we identified two genes, tomato (Solanum lycopersicum) calcium-dependent protein kinase 27 (SlCDPK27; also known as SlCPK27) and its paralogue SlCDPK26, that control fruit sugar content. They act as sugar brakes by phosphorylating a sucrose synthase, which promotes degradation of the sucrose synthase. Gene-edited SlCDPK27 and SlCDPK26 knockouts increased glucose and fructose contents by up to 30%, enhancing perceived sweetness without fruit weight or yield penalty. Although there are fewer, lighter seeds in the mutants, they exhibit normal germination. Together, these findings provide insight into the regulatory mechanisms controlling fruit sugar accumulation in tomato and offer opportunities to increase sugar content in large-fruited cultivars without sacrificing size and yield. A study identifies two genes that act as brakes controlling the sugar content of tomatoes and demonstrates their manipulation to generate sweeter tomatoes without affecting the fruit size and yield.