Biochar amendment has many benefits for improving soil water holding capacity and plant growth particularly under reduced irrigation regimes, yet the underlying biochemical and physiological mechanisms remain largely elusive. The combined effects of biochar addition and deficit irrigation under two N fertilizer regimes on tomato plant growth and physiology were investigated. The results showed that, despite a negative effect on leaf N content, biochar amendment improved the plant water status and leaf gas exchange rates under deficit irrigation, thereby enhanced the biomass (DM) of tomato plants irrespective of N regimes. Even though biochar amendment tended to reduce instantaneous water use efficiency, it significantly enhanced plant water use efficiency (WUEp). Biochar increased soil pH and resulted in an increase in xylem pH, which however did not amplify the root-to-shoot ABA signalling inducing early stomatal closure during deficit irrigation. The principal component analysis (PCA) plot showed that plant water use (PWU) rather than WUEp contributed significantly to the enhanced DM under biochar amendment. Thus, the greater stomatal conductance and transpiration rate and consequently the higher PWU as exemplified by the lowered leaf δ¹³C and δ¹⁸O values might have contributed to the increased DM of the biochar treated tomato plants. It is concluded that biochar amendment could be a promising practice to enhance tomato seedling growth under reduced irrigation and N fertilization regimes.