Photosynthetic stimulation by elevated [CO2] (e[CO2]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N2-fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO2]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO2] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO2] and subjected to well-watered or drought treatments, and then exposed to 13C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased 13C uptake and nodule sink strength, and this effect was even greater under e[CO2], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N2 fixation, and increased accumulation of new photosynthates (13C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO2], and legumes may only be able to achieve greater C gain if nodule activity is maintained.