The effects of water deficits (WD), heat shock (HS), and both (HSWD) on photosynthetic carbon- and light-use efficiencies together with leaf ABA content, pigment composition and expressions of stress- and light harvesting-responsive genes were investigated in ABP9 [ABA-responsive-element (ABRE) binding protein 9] transgenic Arabidopsis (5P2). WD, HS, and HSWD significantly decreased photosynthetic rate (A) and stomatal conductance (g(s)) in wild-type plants (WT). A and g(s) of 5P2 transgenic plants were slightly reduced by a single stress and were hardly modified by HSWD. Although A and electron transport rate (ETR) in 5P2 plants were depressed under optimal growth conditions (control) in relation to WT, they were enhanced under HS and HSWD. These results indicate that ABP9 transgenic plants are less susceptible to stress than the WT. In addition, the increased ABA contents in both WT and 5P2 plants in response to WD and/or HS stresses suggest that declines in A and g(s) might have been due to ABA-induced stomatal closure. Moreover, compared with WT, 5P2 plants exhibited higher ABA content, instantaneous water use efficiency (IWUE), Chl a/b, NPQ, and lower Chl/carotenoid ratios. Finally, altered expression of stress-regulated or light harvesting-responsive genes was observed. Collectively, our results indicate that constitutive expression of ABP9 improves the photosynthetic capacity of plants under stress by adjusting photosynthetic pigment composition, dissipating excess light energy, and elevating carbon-use efficiency as well as increasing ABA content, IWUE, and expression of stress-defensive genes, suggesting an important role of ABP9 in the regulation of plant photosynthesis under stress.