Understanding local distribution and diversity of microplastics (MP) in subtidal sediments is crucial to assess the availability of such particles for ingestion by marine organisms and to identify the potential pollution sources upon which is urgent to act towards the prevention of MP inputs in the region. The strong anthropogenic pressure from Sesimbra and Setúbal municipalities (Portugal), along with the multiple activities (industries, fishing and tourism) taking place on the Arrrábida coast together with the proximity of a marine protected area, makes a relevant case study for MP pollution research. Temporal and spatial distributions of MP on the seabed were assessed monthly at six stations along the Sado river estuary and the Professor Luiz Saldanha Marine Park. Sediment was collected with a Petite Ponar grab, at the 5 m isobath. Granulometry, loss on ignition and MP:meiobenthos ratio was determined for each sample. MP were extracted with a zinc chloride solution (1.5 g cm-3), after removing the organic matter with 10% hydrogen peroxide from each sample. A total of 4738 MP was registered, and 7 types of MP were identified: fibers, fragments, filaments, beads, glitter, films, and tangled ball of fibers. The majority of MP consisted of fibers (44%) and fragments (34%). Preliminary results suggest that MP concentration tends to decrease in stations further away from the Sado estuary (1347.2 ± 804.1 particles kg-1 dry sediment; mean ± SD) where sediments were finer and contained more organic matter; MP concentrations were higher in October (702.8 ± 933.4 particles kg-1 dry sediment; mean ± SD), decreased till January and increased again in February. Beads, glitter, and tangled ball of fibers were rarely found outside of the Sado estuary, suggesting that the finer estuarine sediment may act as a sink for some types of MP.