This study presents baseline data on the physiochemical properties and potential uses of macroalgal (seaweed) biochar produced by pyrolysis of eight species of green tide algae sourced from fresh, brackish and marine environments. All of the biochars produced are comparatively low in carbon content, surface area and cation exchange capacity, but high in pH, ash, nitrogen and extractable inorganic nutrients including P, K, Ca and Mg. The biochars are more similar in characteristics to those produced from poultry litter relative to those derived from ligno-cellulosic feedstocks. This means that, like poultry litter biochar, macroalgal biochar has properties that provide direct nutrient benefits to soils and thereby to crop productivity, and will be particularly useful for application on acidic soils. However, macroalgal biochars are volumetrically less able to provide the carbon sequestration benefits of the high carbon ligno-cellulosic biochars.