This study aimed to investigate the effect of different boron (B) treatments on drought tolerance of watermelon plants. Drought tolerant Citrullus lanatus (Thunb.) Matsum. et Nakai genotype ‘Kar 98’ was grown in controlled greenhouse conditions hydroponically and exposed to drought stress by applying PEG 6000 (polyethylene glycol) in the presence of three boron dosages: 0.05, 0.25 and 1.25 mM. Growth parameters (fresh weight, dry weight and lengths of shoot and roots), leaf relative water content, boron accumulation, lipid peroxidation level and activities of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) enzymes were determined as well as the accumulation of hydrogen peroxide (H<inf>2</inf>O<inf>2</inf>) and hydroxyl (•OH)-scavenging activity were assayed. Increasing dosages of boron alone caused more severe growth reduction than combined with PEG 6000-induced drought stress. Induced drought stress caused less accumulation of boron in leaves and roots. B concentration of 1.25 mM caused lipid peroxidation in a reactive oxygen species-independent manner and drought stress-induced lipid peroxidation was alleviated by increasing B dosages. Induced glutathione reductase activity under the combination of 1.25 mM B and PEG 6000-induced drought stress seemed an important physiological response in ‘Kar 98’ plants against multiple stresses.