Among various environmental factors, the effects of light quality and quantity on growth of vegetables and ornamental plants were investigated thoroughly in closed plant factories, whereas how solutes and minerals in hydroponics affect on growth of vegetables were poorly understood. In this study, we investigated the effects of salinity and nutrients on the growth of red leaf lettuce (Lactuca sativa L. cv. Mother-red). A closed plant factory with hydroponic cultures supplemented with sodium salt (NaCl) or seawater was employed for cultivation of lettuce. The fresh weight of lettuce grown in culture with NaCl was dramatically reduced when compared to those grown in standard culture or culture with seawater. The lettuces grown in the culture with NaCl or seawater contained higher amounts of sugar and anthocyanin, and the lettuce plants grown in the culture with seawater accumulated highest levels of photosynthetic pigments, chlorophylls and carotenoids. These results indicate that the addition of some solutes (e.g. NaCl) to the culture medium affect the size of plant as well as the levels of sugars and pigments in the plant, and suggest that the addition of seawater (20%) to the medium is effective to produce lettuces with higher quality and nutritional value.