Lacui Formation (Spanish: Formación Lacui) is a marine Miocene sedimentary formation located in Chiloé Island with minor outcrops near Carelmapu on the mainland.

[1][2] Gastropod shells are the most common macrofossils of Lacui Formation.

[2] According to Sernageomin (1998) the formation dates to the earliest Serravallian — that is the Middle Miocene.

[2] South of Lacui Formation there are equivalent sedimentary rocks in the islands of Ipún, Lemo and Stoke.

[3]