The effects of extracts of the brown marine alga (Ascophyllum nodosum, ANE) on growth and biochemical and molecular changes in spinach were studied. Overall increases in biomass, chlorophyll, and antioxidant activity were observed at an application rate of 0.1 g L–1 ANE. Shoot fresh weight, dry-matter content, and total soluble protein showed 1.6-, 1.2-, and 1.5-fold increases, respectively. Total chlorophyll increased by 30% and total antioxidant capacity, phenolics, and flavonoid content increased by at least 33%. A 1.4-fold increase in chalcone isomerase activity was observed, whereas the activity of phenylalanine ammonia lyase was not affected. The ANE affected the transcript abundance of genes that affect sucrose and glycine betaine metabolism. The transcript abundance of cytosolic glutamine synthetase (GS1), betaine aldehyde dehydrogenase (BADH), choline monooxygenase (CMO), and glutathione reductase (GR) increased in plants treated with 0.1 g L–1 ANE.