The Portland Group (defining the Portlandian) is a Late Jurassic (Tithonian) lithostratigraphic group (a sequence of rock strata) in South East England.

[1] The name is derived from the Isle of Portland in Dorset where the strata are exposed and have been extensively worked.

Rocks of this age have in the past been called the Portlandian stage by geologists, which corresponds with the late Tithonian stage of the internationally used geological timescale.

The rocks include limestones, dolomitic sandstone, siltstone, and mudstone.

The Portland Sand Formation is made up largely of dolomites but includes siltstones and fine-grained sandstones in its lower parts.