Discharges of contaminants to aquatic environments have increased in recent years and plastic pollution has become a serious concern around the world. However, there is still a knowledge gap on the effects of nanoplastics (NPs) in conjunction with other contaminants. Arsenic (As), a metalloid frequently found in sediments and accumulated in organisms, is among the contaminants which may interact with NPs. Polychaetes are frequently the most abundant group of organisms in benthic communities and, as benthic organisms, are exposed to harmful materials present in sediments. Thus, this study aimed to assess how a parental pre-exposure to 100 nm polystyrene nanoplastics (PS NPs) would influence the response of the polychaete Hediste diversicolor to exposure to As in terms of behaviour, neurotransmission, antioxidant defences, oxidative damage and energy metabolism. H. diversicolor specimens, collected at a reference site, were allowed to acclimate to laboratory conditions. Organisms were then distributed into two experimental groups: group one exposed for 28 days to 0.005 mg/L of 100 nm PS NPs and group two maintained under control conditions without PS NPs exposure, for 28 days. After this period, reproduction was induced and offspring were allowed to grow for six months. Organisms from both groups were then exposed to 0, 50, and 250 µg As/L. Overall, obtained data revealed increased activity of antioxidant enzymes superoxide dismutase and glutathione S-transferases, particularly in parentally exposed organisms. Higher oxidative damage through protein carbonylation and lipid peroxidation was detected in parentally exposed organisms. Cholinesterase activity was inhibited in polychaetes exposed to As, in both groups. These alterations were reflected at the behavioural level, with organisms increasing the time needed to burrow into the sediments, especially those from parental exposure which demonstrated a concentration-dependent increase. Overall, the data suggest that parental exposure to plastics may exacerbate the effects of other contaminants on offspring.