It is situated on the northern coast of France, at the mouth of the Seine, and includes the cities of Rouen and Le Havre.

In 1800 five arrondissements were created within the département, namely Rouen, Le Havre, Dieppe, Neufchatel and Yvetot, although the latter two were disbanded in 1926.

As of 2019, there are 7 communes with more than 20,000 inhabitants:[3] Previously lacking a demonym, the inhabitants of Seine-Maritime (as the department had been renamed in 1955) chose, following a public consultation, to be identified in official documents as "Seinomarins" [8] (males) and "Seinomarines" (females).

The département is connected to the adjacent Eure department via the Tancarville and Pont de Normandie bridge crossings of the Seine.

Cauchois is the dialect of the Pays de Caux, and is one of the most vibrant forms of the Norman language beyond Cotentinais.