Seine-Saint-Denis (French pronunciation: [sɛn sɛ̃ d(ə)ni] ^ⓘ) is a department of France located in the Grand Paris metropolis in the Île-de-France region. In French, it is often referred to colloquially as quatre-vingt treize or neuf trois ("ninety-three" or "nine three"), after its official administrative number, 93. Its prefecture is Bobigny.

In 2019, it had a population of 1,644,903 across 39 communes. In French, the learned but rarely used demonym for the inhabitants of Seine-Saint-Denis is Séquano-Dionysiens; more common is Dionysiens.

The department is surrounded by the departments of Hauts-de-Seine, Val-de-Marne, Paris, Val-d'Oise, and Seine-et-Marne. It is thus one of only five French departments surrounded entirely by other departments of the same region.

Seine-Saint-Denis is located to the northeast of Paris. It has a surface area of only 236 km² (91 sq mi), making it one of the smallest departments in France. Seine-Saint-Denis and two other small departments, Hauts-de-Seine and Val-de-Marne, form a ring around Paris, known as the Petite Couronne ("little crown"). Since 1 January 2016, together with Paris, they have formed the area of Greater Paris (Grand Paris).

The most populous commune is Saint-Denis; the prefecture Bobigny is the eleventh-most populous. As of 2019, there are 5 communes with more than 70,000 inhabitants:

Seine-Saint-Denis is made up of three departmental arrondissements and 40 communes:

Arrondissement of
Saint-Denis:

Arrondissement of
Bobigny: