Neoephemera antiqua is an extinct species of square-gill mayfly in the family Neoephemeridae that is known from early Eocene, Ypresian stage,[1] lake deposits near the small community of Republic in Ferry County, Washington, USA.

[2] The fossil specimen is from the University of Washington site number UWBM A0307B[2] which works sediments from the Tom thumb tuff member[3] of the Klondike Mountain Formation.

[2] Further refinement of the dating has resulted in the formation being given a slightly older age, placing it in the Ypresian stage of the late Early Eocene.

[1][4] Neoephemera antiqua is the oldest known occurrence of the family Neoephemeridae, being older than the Oligocene species Potamanthellus rubiens, described from Montana in 1977 by Standley Lewis.

The full naiad is estimated to have been 8.5 millimetres (0.33 in) in length, and is distinguishable from other species of neoepherids by is shorter pronotum and the structure of the posterior margins of the abdominal segments.