SUMMARY Nectar volume and sugar composition are key determinants of the strength of plant–pollinator mutualisms. The main nectar sugars are sucrose, glucose and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose‐dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa . Moreover, a brcwinv4a null mutant: (i) has greatly reduced CWINV activity in the nectaries; (ii) produces a sucrose‐rich nectar; but (iii) with significantly less volume. These results definitively demonstrate that CWINV activity is not only essential for the production of a hexose‐rich nectar, but also support a hypothetical model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular‐to‐extracellular sucrose ratio that facilitates the continuous export of sucrose into the nectary apoplast. The extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. These results cumulatively indicate that modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition. Finally, honeybees prefer nectars with some sucrose, but wild‐type B. rapa flowers were much more heavily visited than flowers of brcwinv4a , suggesting that the potentially attractive sucrose‐rich nectar of brcwinv4a could not compensate for its low volume.