Seedling production of Undaria pinnatifida using free-living gametophytes has remained at an experimental scale despite the ease of stock preservation and cultivar improvement, due to methodological difficulties during the attachment process of gametophytes to so-called seed strings. The present study tested the feasibility of U. pinnatifida seedling production on a commercial-scale using free-living male/female gametophytes that respectively originated from single zoospores. A new practical method of “gametophyte application”was developed, in which free-living gametophytes are applied to seed strings using a brush. Commercial-scale seedling production and cultivation experiments showed that the new method with free-living gametophytes successfully produced seedlings both in indoor bottles and in outdoor tank culture and these seedlings grew normally. Results indicate that the method has potential for both environmentally stable indoor facilities and also outdoor facilities with low initial cost and labor requirements. These findings show that the new method using free-living gametophytes can be used as an alternative source for seedling production of U. pinnatifida for commercial-scale production..