The root holoparasitic weed sunﬂower broomrape (Orobanche cumana Wallr.) specifically infects sunflowers (Helianthus annuus L.) and causes significant loss of their quality and production. This experiment was designed to evaluate the ameliorative effects of 5-aminolevolinic acid (ALA) on susceptible sunflower (cv. TK0409) under O. cumana stress. Sunflower seeds were pretreated at five concentrations (0, 5, 10, 15, or 20 mg L−1) of ALA, and then the germinated seeds were grown in O. cumana infested and non-infested soil. Results revealed that malondialdehyde (MDA) and reactive oxygen species (ROS) were significantly increased in both roots and leaves along with O. cumana penetration, suggesting oxidative bursts in sunflower, whereas exogenously applied ALA significantly reduced the levels of MDA and ROS by inducing anti- and non-antioxidant enzyme activities under O. cumana stress. Besides, ALA obviously recovered the ultrastructural damages of broomrape-invaded leaf mesophyll and root tip cells in sunflower. Moreover, application of ALA significantly decreased the expressions of xyloglucan endotransglucosylase/hydrolase related genes (XTH6 and XTH9) in sunflower under O. cumana stress compared with the O. cumana infected plants alone, indicating that the increased resistance of sunflower might be related to hypersensitive reactions which were activated by ALA. All the applied dosages of ALA had the effects of enhancing the resistance of sunflower against O. cumana, and the most effective mitigation was observed at 10 mg L−1 ALA pretreatment. In conclusion, ALA enhanced sunflower resistance to broomrape by promoting antioxidant defence system, reducing ROS levels, decreasing cellular damage and regulating the expressions of stress related genes.