Tomato is one of the most important foods and economic crops in Iran and worldwide. Tomatoes require a lot of water to grow well and are adversely affected by drought. Considering the reports published on positive effects of seaweed extract on increasing resistance of plants to abiotic stresses, the present study aimed to evaluate the effects of a seaweed extract (Sargassum angustifolium) on growth and yield indices of tomato under drought stress. This study was conducted as a factorial experiment with three concentrations of algae extract (0, 0.5, 1 g L^-1) and three levels of drought stress (control, 40% FC, 60% FC) plus a non-stress treatment with three replications. To evaluate the effects of algae extract on tomato under drought stress conditions, a combination of morphological and physiological characteristics including plant height, photosynthetic pigment content, lycopene content, phenolic compounds and antioxidant activities were measured. Results showed that treatment by seaweed extract significantly increased plant resistance to drought stress and significantly increased plant height and the content of chlorophyll, phenolics, lycopene, and proline content and significatly improved reactive oxygen species scavenging capacity and superoxide mismutase activity in  tomato. The best results of morphological and physiological traits were obtained using 1 g L^-1 seaweed extract.