The implementation of sustainable agricultural practices, which allow for more efficient utilization of natural resources, as well as reduced pollutant emissions, has become an imperative. Given this context, different cultivation solutions, such as hydroponic methods and alternative fertilizer sources, should be considered. This study evaluated the potential of phytoplankton, Gephyrocapsa oceanica, as a substitute for secondary macronutrients and micronutrients.. Lettuce plants (Lactuca sativa L. var. capitata) were grown in a non-circulating hydroponic system in order to test different nutrient solutions. We assessed four different growing media containing: distilled water without added nutrients, distilled water enriched exclusively with nitrogen, phosphorus, and potassium, distilled water enriched with nitrogen, phosphorus, and potassium, and Gephyrocapsa oceanica extract, and distilled water enriched with traditional inorganic fertilizers. Growth parameters of the treated lettuce, such as the fresh and dry weight of the shoot and roots, head diameter, root length, leaf area, and specific leaf area were determined. Additionally, we evaluated plant composition in terms of micronutrient profile (Ca, Mg, Fe, Mn, Zn) and crude protein and fiber production (neutral detergent fiber NDF, acid detergent fiber ADF, acid detergent lignin ADL, cellulose, hemicellulose). Lettuce plants grown with Gephyrocapsa oceanica extract presented complete development and agronomic parameters, comparable to those of plants cultivated using the conventional nutrient solution. With emphasis to all the parameters, phytoplankton extract result to be suitable for use in hydroponic cultivation and may serve as a promising tool in sustainable agriculture.