The aim of this study was to investigate the biochemical, physiological and morphological responses of oil palm seedlings to mannitol-induced water deficit. Proline content and the percentage of relative membrane leakage in seedlings increased when subjected to water deficit, depending on the degree of water deficit. Relative membrane leakage was positively associated with chlorophyll degradation. Chlorophyll a (Chl_a), chlorophyll b (Chl_b), total carotenoids (C_x+c), maximum quantum yield of PSII (F_v/F_m) and photon yield of PSII ( Φ _PSU) in the seedlings under water deficit conditions dropped significantly in comparison to the control group, leading to a reduction in net photosynthetic rate (P_n) and growth. In addition, the physiological changes and growth parameters of the plants responding to severe water deficit underwent a greater reduction than those of the plants responding to mild water deficit. These data may be applied to establish criteria for water deficit tolerance screening in breeding programmes.