Three lepidopteran species, from the Middle Jurassic Daohugou beds (inner Mongolia, China), are described in a new family, Mesokristenseniidae, and new genus, Mesokristensenia, which could represent the sister group of the Micropterigidae. Mesokristensenia differs from all extant Lepidoptera, but one genus (Agathiphaga, Agathiphagidae), in retaining four median veins in the forewing, a plesiomorphy also present in many Trichoptera. Evidence for placing Mesokristensenia in the Lepidoptera includes four traits, notably a previously unrecorded autapomorphy of this insect order: beyond stem M1+2, vein M1 is bent and connected to cross-vein r-m (in both wing pairs). Among 24 characters taken into account to assess the systematic position of Mesokristensenia, 12 are considered informative for a cladistic analysis involving this fossil taxon and the four suborders recognized in present-day Lepidoptera (Zeugloptera, Aglossata, Heterobathmiina, and Glossata).