The beneficial effects of seaweeds (macroalgae) and their extracts on plant systems has been used for many centuries to improve soil properties and to enhance the productivity and quality of agricultural crops and ornamental plants and turf grass. However, it has only been recently that advances in scientific research have uncovered the potential applications for commercially produced macroalgal extracts as plant biostimulants. Extensive literature on seaweed-derived extracts as biostimulants to improve plant tolerance to abiotic and/or biotic stress, plant growth promotion and their effects on root/microbe interactions, have demonstrated the varied roles in plant health, quality/biofortification and soil improvement. These effects have been supported by physiological, biochemical and molecular mechanisms, as evident through investigations using model plants. This review discusses selected aspects of the role of seaweeds and their extracts on plant growth; it also provides an overview of mechanisms of activity and potential bioactive components. The review is presented to stimulate further studies on the use of macroalgal-derived extracts for applications in agriculture and horticulture.