In the Russian industry, the main components extracted from brown seaweedsare mannitol, alginic acid, and its salts. Other seaweed components are currently classified as waste. However, by-products of brown seaweed processing, as demonstrated by global practice, are quite suitable for further biotechnological processing and use. The effect of kelp extract (industrial waste, specifications TU 10.39.30-025-41669896-2019) on seed germination and growth of seedlings in cucumber and wheat, used as test objects, was studied in a controlled environment, both under optimal conditions and under salt stress (NaCl 150 mmol). Laminaria digitata (Huds.) Lamouroux were collected in waters of the Solovetsky Archipelago in the White Sea. The experimental results indicate a potential biostimulating effect of the extract on plants. The effect was evident at low concentrations (0.25–10 ml/L) and depended on the method of application (seed priming, addition to the seed germination medium, or hydroponic growing solution). Higher concentrations (50 ml/L and above) caused significant inhibition of plant growth. Possible causes of the stimulating and inhibiting effects of kelp extract on seed germination and plant seedling growth are discussed.