Since the late 19th century, studies of the genus Porphyra sensu lato (Bangiales, Rhodophyta) have revealed a broad spectrum of life history strategies and reproductive modes. These variations are often interpreted as adaptive responses to the selective pressures imposed by spatially and temporally heterogeneous environments. However, the dynamics and prevalence of sexual versus asexual reproduction in natural populations remain poorly understood and insufficiently documented. In this study, we investigated the spatio-temporal variation in reproductive strategies of Porphyra umbilicalis, an emerging target species for aquaculture in the Northeast Atlantic, across a broad European range. Through microscopic examination and tracking the germination of spores released from fertile individuals in multiple populations, four main patterns were identified: (1) clear geographic structuring of reproductive strategies, with asexual reproduction prevailing in the Southern Bight of the North Sea and English Channel, and sexual populations more common in the South European Atlantic Shelf and Celtic Seas; (2) persistent dominance of asexual reproduction in populations with mixed reproductive modes, with a seasonal increase in the frequency of sexually reproductive individuals during winter; (3) microhabitat partitioning within the high intertidal zone, with asexual individuals occupying uppermost elevations and sexual individuals concentrated below, indicating ecologically driven niche differentiation; and (4) no consistent link between thallus size and reproductive mode, although sexual individuals were significantly larger during winter and spring. Collectively, these findings illustrate how environmental gradients shape reproductive strategies in P. umbilicalis, and suggest that asexuality may confer adaptive advantages in marginal intertidal zones. Reproductive strategies show clear geographic structuring along the Northeast Atlantic.Asexual reproduction predominates year-round in populations with mixed reproductive strategies.Ecological niche differentiation occurs between sexual and asexual gametophytes. Reproductive strategies show clear geographic structuring along the Northeast Atlantic. Asexual reproduction predominates year-round in populations with mixed reproductive strategies. Ecological niche differentiation occurs between sexual and asexual gametophytes.