Invasive species represent a risk to natural ecosystems and a biodiversity hazard. The present work aims to determine the antioxidant enzyme response - superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX), the phase II detoxifying enzyme - glutathione S-transferase (GST) - and markers of oxidative damage - thioredoxin reductase (TR) and malondialdehyde (MDA) - in gills and digestive gland of Pinna nobilis and to study the antioxidant response effects in the bivalve colonised by the invasive macroalgae Lophocladia lallemandii. Colonised specimens were collected in a control area without L. lallemandii and another area completely colonised by L. lallemandii. All enzyme activities were found to be present in gills and digestive gland, with some tissue differences. CAT and SOD activities were higher in gills than digestive gland, whereas GST activity and MDA levels were higher in digestive gland. The presence of L. lallemandii induced a significant increase in the activities of antioxidant enzymes in both gills and digestive gland, except for CAT activity in gills. GST and TR activities were also increased in both tissues, as well as the MDA concentration. We can conclude that the presence of L. lallemandii colonising P. nobilis induces a biological stress and oxidative damage to the fan mussel.