The Tielt Formation (Dutch: Formatie van Tielt; French: Formation de Tielt; abbreviation: Tt; named after the town of Tielt in West Flanders) is a geologic formation in the subsurface of Belgium.

The formation crops out in the north of Hainaut, in the southern and central parts of West- and East Flanders and in Walloon and Flemish Brabant.

It consists of marine very fine sand and silt, deposited in the shallow sea that covered Belgium during the middle and late Ypresian age (early Eocene, about 53 million years ago).

The formation lies stratigraphically on top of the Kortrijk Formation (early Ypresian sandy clay and silt).

In the northwestern part of Belgium, the Gentbrugge Formation (late Ypresian marine clay and silt) covers the Tielt Formation.