Algal bioproducts are of growing interest to agriculture because of their biodegradable nature, ability to restore soil fertility, and capacity for plant growth regulation, nitrogen fixation, and carbon sequestration. Plants respond to a suite of growth hormones; auxins present in algal extracts or secreted exogenously by living algae may be partially responsible for the stimulation of plant growth. Auxins are a major class of phytohormones that influence plant growth and development. The roles of auxins in algae and in plants are well described, but studies on the role of auxins in plant-algae interactions remain scarce. This review summarizes the body of knowledge on the production of auxins and their phsiological roles in seaweeds, cyanobacteria, and microalgae. Common and differential auxin-associated phenotypes of these algae, including the effect of growth conditions on their auxin production, are also described. Potential mechanisms by which auxins from algae mediate plant development at both phenotypic and molecular levels are also provided. Algal-derived auxins are an environmentally sustainable option for promoting plant growth and yield, but knowledge of their precise mechanisms of action is still rudimentary. Elucidating the pathways by which algal auxins stimulate plant responses and the means by which key environmental factors influence those pathways will help to harness the full potential of algal-derived auxins for agricultural development and resource conservation.