Tiaojishan Formation, Geological formation in Hebei, Inner Mongolia, and Liaoning, China.

The Tiaojishan Formation is a middle-to-late Jurassic geological formation in Hebei, Inner Mongolia and Liaoning, China. It consists mainly of pyroclastic rock interbedded with basic volcanic and sedimentary rocks, and was formerly grouped with the underlying Haifanggou Formation as the "Lanqi Formation." Its tuffaceous layers record periodic heavy ashfalls, and its fossils include plants, insects, vertebrates and the gliding mammal Volaticotherium.

The Tiaojishan Formation and the underlying Haifanggou Formation were once grouped as a single "Lanqi Formation". Argon-argon dating by Wang and colleagues in 2005 gave about 160 Ma for part of the formation; U-Pb zircon work by Liu and colleagues in 2006 correlated it with the Daohugou Beds, and a 2008 study placed its base at 165 Ma and its top between 156 and 153 Ma.

The formation's name comes from Tiaojishan in Hebei; its fossil-bearing beds are best known through the Daohugou locality in Ningcheng County, Inner Mongolia, where the Yanliao Biota was first recognised. Fieldwork there since the early 2000s has yielded feathered dinosaurs, the gliding mammal Volaticotherium and the swimming mammaliaform Castorocauda, and the Daohugou beds are now placed within this formation.

This rock formation is spread across a large area, so visitors can explore various outcrops and fossil sites at multiple locations throughout the region. It helps to learn about specific sites before visiting since the exposures are not concentrated at one central point.

The Daohugou Beds were long placed in the underlying Haifanggou Formation, and estimates of their fossils ranged from 169 to 122 million years. Argon-argon dating in 2012 gave 164 ± 4 million years, fixing them within the Tiaojishan Formation. The same beds yielded Castorocauda, a swimming mammaliaform with a beaver-like tail.