Several studies have identified low soil P and water availability as major constraints to pearl millet [Pennisetum glaucum (L.) R. Br.] production in semi‐arid West Africa. To evaluate the effects of phosphate and water supply on yield, transpirational water‐use efficiency (WUET), and carbon‐isotope discrimination (Δ), two varieties of pearl millet were cultivated in pots in a glasshouse at the ICRISAT Sahelian Centre, near Niamey, Niger. Phosphate and water supply had significant effects on yield, WUET, and Δ. Compared with the control plants, which had adequate water and P availability, yield was reduced 34% by low water supply and 48% by low P supply. Under high P‐supply, water stress increased WUET by approximately 37%. Under low P‐supply, no effect of water supply on WUET was observed. Water stress increased Δ by approximately 0.6‰ for low P plants, and 0.9‰ for high P plants. Added P increased Δ by 0.3 to 0.4‰. WUET and Δ did not differ significantly between varieties. Differences in Δ between green and necrotic leaves were found within both P treatments under low water supply. We attribute changes in Δ to changes in the ratio of external to internal concentration of CO2, (pi/pa), leakage rates of CO2 out of bundle‐sheath cells, respiration rates, or chemical composition of the plant material.