Marine seaweeds are nature’s largesse resource in human well-being. Seaweed’s contribution to the food industry is being widened for its rich source of essential vitamins, minerals, and PUFA. ‘Nori’ is a dried sheet of red seaweed Porphyra sp. used to make ‘sushi’ a renowned cuisine in Japan, and other Southeast Asian countries. In this present study, commercial-grade ‘Nori’ was extracted and fractionated with different solvents, and subjected to evaluate the cytotoxic activity on human ovarian tetracarcinoma cells (PA1). As a result, the ethyl acetate fraction was found effective in inhibiting the proliferation of human ovarian cancer cells with an IC50 value of 41.1 µg/mL. The main component responsible for anticancer activity was determined as a sesquiterpene lactone Tatridin B (3h-cyclodeca[b]furan-2-one, 4,9-dihydroxy-6-methyl-3,10-dimethylene-). Moreover, it was found effective in inhibiting NSD2, an important and recently proposed biomarker for several types of cancer. This is a hitherto report on the presence of this compound in ‘Nori’ showing potent anticancer activity on human ovarian tetracarcinoma (PA1).