Vegetarians are likely to have lower intakes of preformed docosahexaenoic acid (DHA) than omnivorous populations who consume fish and animal products. As such, vegetarian populations have omega-3 indices up to 60% lower than those who consume marine products. Algae, the primary producer of DHA in the marine food chain, offer an alternative source of DHA for those who do not consume marine or animal products. This systematic review aims to examine the evidence for the relationship between supplementation with algal forms of DHA and increased DHA concentrations in vegetarian populations. The SCOPUS, Science Direct and Web of Science scientific databases were searched to identify relevant studies assessing the effect of algal DHA consumption by vegetarian (including vegan) populations. Four randomized controlled trials and two prospective cohort studies met the inclusion criteria. All included studies reported algal sources of DHA significantly improve DHA concentrations (including plasma, serum, platelet, and red blood cell fractions) as well as omega-3 indices in vegetarian populations. An evident time or dose response was not apparent given the small number of studies to date. Future studies should address long chain n-3 polyunsaturated fatty acid deficiencies in vegetarian populations using algal DHA and explore the potential physiological and health improvements in these individuals.