The present study evaluated the biostimulating effect on seed germination and initial vegetative development of Phaseolus vulgaris L. bean seedlings in response to the application of the aqueous extract of Osmundaria obtusiloba (C. Agardh) RE Norris (Rhodophyta) and Sargassum vulgare C. Agardh Ochrophyta), relating the activity with the contents of proteins and carbohydrates of extracts. Seaweed dried biomass of macroalgae were extracted at the concentration of 500 g L-1 in distilled water, this concentration was considered 100%. After was prepared in dilutions of 25%, 50%, and 0% (negative control) for each species. The experiment was conducted with 140 bean seeds in transparent plastic boxes kept in incubators under a12 hour photoperiod at the temperature of 25°C for 15 days. During this time, the germination percentage, germination speed and vigor of the seedlings (shoot length x radicle length) were assessed, as well as the sugar and total protein content of each seaweed extract. The results indicated that the development of seedling aerial biomass was 1.8 times higher with the extract at 25% concentration of O. obtusiloba than with the negative control. There were no changes in the germination parameters. Therefore, the O. obtusiloba extract showed a more positive effect on initial development than that of S. vulgare. The O. obtusiloba extract contained 160 mg L-1 while that of S. vulgare contained 80 mg L-1 total soluble proteins, indicating that protein content might be indicative of seaweed potential for application in agriculture.