A mastodon tooth (cf. Tetralophodon longirostris) was collected in marine deposits exposed at Penedo Norte beach section in the Setúbal Peninsula. This finding allowed its accurate dating. It was found in bed 8 of this section, T1 depositional sequence that dates from basal Tortonian, ca. 11.6 Ma after N14 planctonic forams and 87Sr/86Sr and K/Ar ages. The concerned tooth seems to belong to Tetralophodon longirostris, a species already known in Portugal in continental deposits from the first part of Upper Miocene.