سن-سن-دونی (به فرانسوی: Seine-Saint-Denis) نود و سومین شهرستان فرانسه است . شهرستان سن-سن-دنی در شمال فرانسه قرار دارد. این شهرستان از سمت شمال و شمال شرق پاریس با این شهر همسایه است. این شهرستان زیرمجموعه ناحیه ایل-دو-فرانس میباشد. با وجود این که مساحت این شهرستان فقط ۲۳۶ کیلومتر مربع میباشد ، جمعیت آن بالغ بر ۱٬۵۰۲٬۳۴۰ نفر میباشد . شهرستانهای سن-سن-دنی، وال دو مارن و او-دو-سن سه شهرستان کوچک فرانسه هستند که در مجموع شهر پاریس (شهرستان پاریس) را احاطه کردهاند. 
Seine-Saint-Denis (French pronunciation: [sɛn sɛ̃ d(ə)ni]) is a French department located in the Île-de-France region and in the Grand Paris. Locally, it is often referred to colloquially as quatre-vingt treize or neuf trois (i.e., "ninety-three" or "nine three"), after its official administrative number, 93.
The learned and rarely used demonym for the inhabitants is Séquano-Dionysiens; more common is Dionysiens.
Seine-Saint-Denis is located to the northeast of Paris. It has a surface area of only 236 km², 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 form the area of Greater Paris.
Seine-Saint-Denis was created in January 1968, through the implementation of a law passed in July 1964. It was formed from the part of the (hitherto larger) Seine department to the north and north-east of the Paris ring road (and the line of the old city walls), together with a small slice taken from Seine-et-Oise.
Seine-Saint-Denis has a history as a veritable left-wing stronghold, belonging to the ceinture rouge (red belt) of Paris. The French Communist Party especially has maintained a continued strong presence in the department, and still controls the city councils in cities such as Saint-Denis, Montreuil and La Courneuve. Until 2008, Seine-Saint-Denis and Val-de-Marne were the only departments where the Communist Party had a majority in the general councils but the 2008 cantonal elections saw the socialists become the strongest group at the Seine-Saint-Denis general council (while the Communist Party gained a majority in Allier and lost it in 2015).
A commune of Seine-Saint-Denis, Clichy-sous-Bois, was the scene of the death of two youths which sparked the nationwide riots of autumn 2005. In October and November, 9,000 cars were burned and 3,000 rioters were arrested.
Seine-Saint-Denis is the French department with the highest proportion of immigrants: 21.7% at the 1999 census (see table below). This figure does not include the children of immigrants born on French soil as well as some native elites from former French colonies and people who came from overseas France. The ratio of ethnic minorities is difficult to estimate accurately as French law prohibits the collection of ethnic data for census taking purposes.
In 2018, the poverty rate was twice the national average at 28%, the unemployment rate was 3 percentage above the national average and 4 percentage points above the Île-de-France average at 12.7%. In 2018, it was estimated that 8-20% of the population in the department were illegal immigrants.
Population development since 1881:
An education study confirmed falling levels of literacy in the area, where the fraction of pupils who had 25 errors or more increased from 5.4% in 1987 to 19.8% in 2015.
Place of birth of residents
Current National Assembly Representatives