Lessonia trabeculata is a brown seaweed inhabiting the rocky subtidal zone along the coast of central and northern Chile, where it is the dominant kelp, and an important species in community structure. Morphological and reproductive aspects of this alga are dependent on environmental conditions and geographic distribution, and the present study gives data on its reproductive periodicity. The reproductive phenology for three populations from northern Chile (29–30°S) was evaluated by means of seasonal examination of morphological and reproductive characteristics of both macroscopic sporophytes and microscopic gametophytes. Comparative laboratory cultures of spores were made to determine seasonal differences in their capacity to produce viable plants. This species is perennial, and demonstrates year‐round presence of reproductive tissues, although showing variation in reproductive phenology over time and among populations. The size of blades increases in spring and summer, whereas its reproductive potential (e.g., area and proportion of the reproductive tissue and the release of spores) increases in autumn. Culture experiments showed that spring and summer reproductive tissue released zoospores which had low germination rates compared to those of autumn and winter, and which produced female gametophytes of low fertility. The population differences depended on the character analysed and the season of the year. The development of both phases of the life cycle of L. trabeculata may be influenced by the local environmental conditions and their seasonal changes, and were expressed as morphological and/ or reproductive changes in the plants. A better understanding of the seasonal adaptations may be obtained if future comparisons are made between widely separated populations or between individuals from the extremes of geographic distribution of the species.