Alaria esculenta is an important kelp species in northern Europe, Atlantic Canada and USA and the Arctic, with high economic potential. Microspongium alariae, a brown algal endophyte using A. esculenta as host, is reported for the first time from Scotland (Great Britain) and Brittany (France), suggesting a wide distribution in NW Europe. The alga was found growing epi-endophytically in A. esculenta stipes and was occasionally associated with warts. Isolated Microspongium thalli grew in host-free cultures and formed plurilocular sporangia in a broad range of temperature and irradiance conditions. DNA barcoding using the nuclear ribosomal ITS1, the mitochondrial COI and the plastidial rbcL confirmed the identity of the endophyte as M. alariae. Electron microscopy was used to compare the alga when endophytic in Alaria with a host-free culture. As an endophyte, cell diameter, pyrenoid diameter and cell wall thickness were reduced. In contrast, there were more plasmodesma connections between endophyte cells, possibly to enhance nutrient transport along the endophytic thallus. In the light of this evidence, a parasitic life style is considered unlikely for the species and the adaptive value of endophytism in M. alariae remains to be elucidated.