Triel-sur-Seine (French pronunciation: [tʁijɛl syʁ sɛn], literally Triel on Seine) is a commune in the Yvelines department in the Île-de-France in north-central France.

It is positioned approximately 11 km (6.8 mi) to the north-west of Saint-Germain-en-Laye.

The city is known for the "Fête du Flan", literally Flan Party, which takes place every last weekend of September on the banks of the Seine.

The bakeries of the city have on this occasion several stands on which it is possible to consume flan.

The city entered the Guinness Book in 1995 for having made the largest flan in the world (5.10 m in diameter).