Seaweeds are green, brown and red marine macroalgae. Extracts of brown seaweeds are widely used in horticulture crops largely for their plant growth-promoting effects and for their ameliorating effect on crop tolerance to abiotic stresses such as salinity, extreme temperatures, nutrient deficiency and drought. The chemical constituents of seaweed extract include complex polysaccharide, fatty acids, vitamins, phytohormones and mineral nutrients. Recent researches have shed light on the possible molecular mechanisms activated by seaweed extracts. In this review we give an update of the current state of our understanding of the chemical constituents of brown seaweed extracts and the physiological effects they induce on plants with particular reference to horticultural crops.