The contamination of deep marine ecosystems is an issue of growing concern. Several studies showed how microplastics (MPs) with a density higher than seawater can sink and accumulate in marine bottom sediments. Polyethylene terephthalate MPs (PET-MPs) represent one of the most common plastic items sinking in deep marine ecosystems worldwide, but to date the information concerning the fate and toxicity towards benthic organisms is still scant. This study investigated the ingestion/egestion and the alteration of oxidative status (i.e., the amount of pro-oxidant molecules (ROS), the activity of antioxidant (SOD, CAT and GPx) and detoxifying (GST) enzymes, and lipid peroxidation) induced by the exposure to irregular shaped PET-MPs in two benthic organisms with different feeding strategies, a filter feeder, the Manila clam Ruditapes philippinarum and a grazer, the sea urchin Paracentrotus lividus. Seven day exposures where performed to administer to clams two concentrations of PET-MPs (0.125 and 12.5 µg/mL), while sea urchins were dietary exposed to three amount (8, 80 and 800 particles/g of food) of PET-MPs. Our results showed that both Manila clams and sea urchins were able to ingest and egest PET-MPs and that their transit throughout the digestive system did not cause tissue alterations to the digestive tract of both species. At biochemical level, PET-MPs induced an oxidative stress situation in gills, but not in the digestive gland, isolated from treated Manila clams. A slight overproduction of pro-oxidant molecules and a modulation of antioxidant enzymes were noted in the proximal part of sea urchin oesophagus. These results showed that the exposure to sinking, irregular shaped PET-MPs might induce the onset of an oxidative stress situation and represent a threat to marine benthic organisms regardless of their feeding strategies.