A palynological analysis with marine palynomorphs (dinoflagellate cysts, acritarchs, green algae) and terrestrial palynomorphs (pollen and spores) of the Kasterlee and Poederlee formations provides new insights in the depositional history at the southern border of the North Sea basin (northern Belgium) around the Miocene–Pliocene transition. Dinoflagellate cyst stratigraphy constrains the age of the Kasterlee Formation in the Oud-Turnhout borehole between 7.5 and 5.32 Ma. The upper boundary of the formation can be correlated with sequence boundary Me2 at 5.73 Ma of Hardenbol and co-workers, which further constrains its age to the time interval 7.5–5.73 Ma. The palynomorph assemblages reflect a near-coast depositional environment. Where present, the Kasterlee Formation thus terminates the Miocene series in northern Belgium. The overall shallow nature of the latest Miocene deposits is related to a sea-level lowering caused by the onset of globally cooling conditions. For the first time, palynology is applied to estimate the age of the Poederlee Formation, suggesting it was deposited during the Mid-Pliocene warm period. Dinoflagellate cysts and sequence stratigraphy together constrain the age of the unit between 3.21 and 2.76 Ma, and possibly even between 3.21 and 3.15 Ma. The Poederlee Formation was deposited in neritic environments, which shoaled in the upper part of the unit as a consequence of the decreasing availability of accommodation space. We demonstrate that the magnitude of the hiatus between the Miocene and Pliocene series varies strongly at the southern boundary of the North Sea Basin, and lasts in the Antwerp area c. 3.2 million years and c. 2.52 million years in the Campine area.