Saccharina sessile is a kelp of the Order Laminariales and thus has a similar life cycle to most kelps with a heteromorphic alternation of generations and a dominant macroscopic sporophyte and a microscopic gametophyte phase.

The sporophytes are large and parenchymous with an intercalary meristem. The gametophytes are microscopic and filamentous with an apical meristem.

S. sessile is oogamous with an egg that is sometimes flagellated and the reproductive unilocular sporangia on the sporophyte are concentrated into patches, or sori (9). It becomes fertile in the late fall and winter with maximum sporophyte growth during the summer (10).