The bacterium Rhodopseudomonas palustris is a microorganism able to fix CO2 and absorb sunlight and, therefore, can increase plant photosynthetic activity and mitigate the effects caused by abiotic stresses. Therefore, this work aimed to evaluate the effect of R. palustris on gas exchange and nitrate reductase activity of mango cv. 'Keitt' grown under tropical semiarid environmental conditions. The experiment was carried out simultaneously in two orchards with the same mango cultivar and crop management practices in Petrolina, Pernambuco, Brazil. The study followed a randomized block design with treatments distributed in a factorial arrangement (7 x 8), referring to different application methods of R. palustris: T1) control treatment; T2) 1.43 x 10(7) CFU/plant via fertigation; T3) 2.85 x 10(7) CFU/plant via fertigation; T4) 4.27 x 10(7) CFU/plant via fertigation; T5) 5.70 x 10(7) CFU/plant via fertigation; T6) 1.43 x 10(7) CFU/plant via fertigation +1.43 x 10(7) CFU/plant via leaf spray; T7) 2.85 x 10(7) CFU/plant via fertigation +1.43 x 10(7) CFU/plant via leaf spray; and evaluation at 0, 30, 60, 90, 120, 150, 180, and 210 days after application of R. palustris. The variables analyzed included: net photosynthesis, internal CO2 concentration, stomatal conductance, transpiration rate, water-use efficiency, and nitrate reductase activity in leaves and roots. R. palustris affects gas exchange and nitrate reductase activity in mango cv. 'Keitt' grown in Brazilian semiarid, but the effects depend on the phenological phase evaluated. The application of R. palustris following 1.43 x 10(7) CFU/plant via fertigation +1.43 x 10(7) CFU/plant via leaf spray were found to increase the photosynthetic activity of mango.