Titanium dioxide nanoparticles (TiO2NPs) generally have positive effects on growth and physiological parameters in plants, but their impacts on photosynthesis and antioxidant system are poorly understood. This work aimed to assess the effects of TiO2 NPs on photosynthetic performance and antioxidant system in Raphanus sativus L. plants. Treatments consisted of TiO2 NPs and microparticles (μPs) applied foliarly by increasing doses from 10 to 1500 mg L−1, using deionized water as a control. As expected, Ti concentration in leaf tissue increased concomitant with the applied dose, being higher in μPs than NPs. A transient increase in net photosynthetic rate (by about 50 %) and stomatal conductance were observed from 12 to 60 h after the beginning of NPs and μPs treatments compared to the control. These increments were independent of doses and particles size. The photochemical parameters were more influenced by the time than the doses, being electron transport rate (ETR) increased, and non photochemical quenching (NPQ) decreased. The instantaneous water use efficiency (WUEins) was 2-fold higher, and intrinsic water use efficiency (WUEint) was 70 % lower in the treated plants respect to the control. Total phenols concentration increased with increasing doses of NPs and μPs. It is suggested that TiO2 NPs and μPs had a transient positive impact on photosynthesis likely by diffusive effects, increasing the WUEins, and a slight activation of secondary metabolism.