This study investigated the effects of drought stress on the growth, physiological, and biochemical parameters of four Petunia cultivars: 1. Iranian Petunia (P1), 2. Hybrid Supercascade White Petunia (P2), 3. Hybrid Grandiflora Frost Blue Petunia (P3), and 4. Hybrid Grandiflora Crimson Star Petunia (P4) under different levels of water deficit (90%, 60%, and 30% of field capacity). The experiment was conducted in a factorial design with four replications in a completely randomized layout. Results showed that drought stress significantly affected the fresh and dry weight of stems and roots. With increased drought stress intensity, the fresh and dry weight of stems and roots decreased across all cultivars. P4 and P3 showed the highest reduction in shoot fresh and dry weight (58.8% and 49% fresh weight reduction, and 48.5% and 44.7% dry weight reduction, respectively) and root fresh and dry weight (45.1% and 35.5% fresh weight reduction, and 51.3% and 47.6% dry weight reduction, respectively) under severe drought stress, whereas P1 recorded the highest dry weight of shoots and roots under severe stress. Stem height and the number of lateral branches were also affected by drought stress. P1 and P2 exhibited a significant reduction in stem height under severe stress (40.4% and 43.3%, respectively). Additionally, the number of lateral branches increased in P1 and P3 under moderate stress (24.4% and 42.9%, respectively), but decreased significantly under severe drought. Drought stress significantly reduced root diameter in P3 (48%). Severe water deficit (30% field capacity) markedly decreased flower diameter across all cultivars, with the largest reduction (22.97%) observed in P4. Furthermore, severe drought stress reduced flower number in all cultivars, with P1 producing the highest flower count under 90% field capacity, though severe drought caused a 72.3% reduction in flower number in this cultivar. Electrolyte leakage and relative water content (RWC) of leaves were affected by drought stress. Electrolyte leakage increased under severe drought (30% field capacity), with P4 showing the highest leakage (36.1%). RWC decreased under water stress, with P4 showing the greatest reduction (24.4%). P1 exhibited the highest proline content (2.24 μmol/g FW) under severe stress. Total carbohydrate content increased under severe stress in P1 and P2 (21.6% and 19.5%, respectively). Photosynthetic pigments were also affected by drought, with chlorophyll a and b content decreasing under water deficit. P4 showed the lowest chlorophyll b content (0.53 mg/g FW) under severe stress. Antioxidant enzyme activities, including SOD, CAT, and POD, responded differently to drought stress. SOD activity increased under moderate stress in P1, P2, and P3 (34.45%, 52.5%, and 24.9%, respectively) but did not show significant changes under severe stress. CAT activity increased in P1 (29.5%) while it decreased in the other three cultivars. POD activity was highest in P1 under non-stressed conditions (5.65 units/g FW) and also increased (9.6%) under drought stress. Pearson correlations revealed a positive correlation between proline and total carbohydrate content with antioxidant enzyme activities, while chlorophyll a and b were closely related. Overall, P1 was identified as the most drought-tolerant cultivar, followed by P2 and P4 in terms of drought tolerance