Oceans and seas form a large body of water that contains a natural biodiversity. For humans, represents a resource, which makes this a point of interest, from researches to improve the economy. Seaweeds produce many compounds and secondary metabolites that can be used in different fields of industry such as food, agricultural, pharmaceutical and health. Even though seaweeds are ancestral resources, recently it was notorious a global interest in knowing more about its potentials, where biotechnology plays an important role in research. Studies showed that seaweed has many bioactive compounds beneficial to plant development, giving them a great potential as an agricultural fertilizer. Adding seaweeds to the soil provides organic matter, minerals, trace elements, growth plant regulator, metabolites, vitamins, and amino acids and it can work as a soil conditioner. In Portugal, the use of seaweeds for agriculture is important since long time ago. In the past, populations that lived near coastal zone depended on the seaweeds as a family subsistence but, throughout the years, synthetic fertilizers replaced seaweeds. Our work aimed to assess the potential of the extracts obtained from Ascophyllum nodosum and from Sargassum muticum as an agricultural fertilizer. This evaluation was carried out with rice plants (Oryza sativa) and lettuce (Lactuca sativa), in germination bioassays, the culture of rice and lettuce plants in pots, and culture of lettuce plants in hydroponics. For that, seaweed liquid extracts were used in different concentrations in different bioassays. Results show that extracts obtained from two seaweeds, A. nodosum and S. muticum, can be promissory plant biofertilizer at a concentration of 25% and had a positive effect on seed germination, plant development, and production.