The gills of farmed Atlantic salmon are susceptible to damage from various stressors, including toxins released by harmful algae (HA). Microcystin is a HA-toxin that affects fish and typically induces toxicity through oxidative stress with detoxification mainly occurring in the liver by glutathione (GSH). While the effect of microcystin on liver is well studied, its impact on gills, which are the first point of contact with microcystin has been overlooked. In this study, we explored the toxic effects of microcystin-LR (MC-LR) on the Atlantic salmon gill epithelial cell line (ASG-10) and whether N-acetyl-l-cysteine (NAC), a precursor of GSH, decreases these potential impacts. The ASG-10 cells were exposed to different concentrations of MC-LR (0, 5, 10, 20, 40 µg/ml) in presence or absence of NAC (1 mM). Cell viability, reactive oxygen species (ROS) production, GSH level, and cell migratory behaviour/wound healing, were measured at different time points for up to 5 days post-exposure. Cell death was only significantly increased after treatment with 40 µg/ml MC-LR after 5 days. There was a significant negative impact of MC-LR on proliferative and migratory ability of cells and the gap in the cell monolayer did not close when treated with 20 and 40 µg/ml of MC-LR. Interestingly, MC-LR had no impact on ROS production and GSH level in ASG-10 cells. However, NAC treatment was associated with reduced ROS and increased intracellular GSH as expected. These data indicate that MC-LR has negative effects on cell proliferation and may impair gill tissue healing and regeneration following damage.