This work reports on the production and characterization of agar-based extracts from the seaweed Gelidium
sesquipedale by means of simple protocols based on hot water and sonication treatments. The combination of
sonication with the hot water treatment reduced 4-fold the extraction time without significantly affecting the
extraction yield (ca. 10–12%) and the extracts' properties. Apart from agar, the extracts contained proteins,
polyphenols and minerals, which conferred them high antioxidant capacity and led to the production of
brownish softer gels. The application of an alkali pre-treatment yielded almost pure agars, with higher molecular
weights and crystallinities than commercial agar, resulting in stiffer gels. However, the partial digestion of agar
by the alkali led to low extraction yields (ca. 2–3%). These results show the efficiency of the combined heat and sonication method to generate cost-effective agarbased extracts with potential applications within the food industry.