Interactions between biogenic monoterpenes and drought stress remain poorly understood and characterised. Even the nature of the response of biogenic monoterpene emissions to water limitation is controversial, possibly depending on the severity, intensity and duration of the drought. Whether monoterpenes regulate plant physiological response to drought stress is currently unknown. In this research, 6-week-old Ailsa Craig wild-type (WT) and ABA-deficient (notabilis) tomatoes were either well-watered or exposed to deficit irrigation (by watering pots with 25% of daily evapotranspiration) in a factorial combination with selected-monoterpenes applied exogenously as a foliar spray. Both genotypes showed similar physiological and biochemical responses to water deficit. Compared to well-watered controls, drought stress significantly reduced net photosynthesis rate and stomatal conductance, increased hydrogen peroxide (H2O2) and malondialdehyde (MDA) concentrations, and also significantly inhibited PSII maximum (Fv’/Fm’) and operating (φPSII) efficiency under severe stress. Drought stress significantly increased foliar abscisic acid (ABA) accumulation in WT plants, whereas notabilis remained ABA-deficient. Applying exogenous monoterpenes decreased net photosynthesis and stomatal conductance of WT plants under moderate drought conditions. Although foliar H2O2 content (a proxy of oxidative stress) was not affected by exogenous monoterpenes, their application significantly decreased the production of MDA (which indicates damage caused by drought-induced oxidative stress). The monoterpene spray also significantly inhibited ABA accumulation under severe stress, possibly by interfering with the methylerythritol (MEP) pathway and thereby reducing production of ABA precursors. Although exogenous monoterpenes increased plant antioxidative capacity by reducing lipid peroxidation, this did not appear to protect photosynthetic activities as the PSII efficiencies or net photosynthesis rate were not affected. That these effects were not observed in notabilis suggests that monoterpenes have ABA-dependent impacts on plant photosynthetic biochemistry.