Marine macroalgae, Kappaphycus alvarezii has economic importance which is considered as the main source of Kappa carrageenan, a hydrocolloid used in various industries as an emulsifying agent and stabilizer. In the production process of this raw material, the fresh seaweed is subjected to grinding and filtering process, yielding a wet solid fraction rich in carrageenan and other liquid fractions. The latter has been studied for its use in pulverization of bean leaves, wheat, soybean, rice, among others. Several reports have shown positive effects on the growth, yield and quality of vegetables and grains treated with the extract of this alga. However, there are few reports on the use of the liquid extract of K. alvarezii for seed treatment, and also of its use in conjunction with the solid fraction of seaweed. In this context, this study aims to evaluate the physiological quality of soybean seeds treated with liquid extract of this alga and, with the mixture of liquid extract and solid biomass, submitted to a hydrolysis process. Two soybean seed treatment tests were performed in laboratory: one treatment contained pure liquid extract (100%) while the other was mixed with two samples of solid biomass hydrolyzed with concentrations of 25 and 75%. For comparison of their results, there was a positive control treatment and one negative control treatment. The results showed that, within the present working conditions, treatments with seaweed extract can increase the length of the aerial part of the seedling up to 28% and the length of the primary root up to 19% as compared to the negative control; thus, it improves the physiological performance of seeds.