A new fossil of the rare and little known arthropod group Chasmataspida (Chelicerata), is described as Achanarraspis reedi gen. et sp. nov. from the Middle Devonian (late Eifelian) of Achanarras Quarry, Caithness, Scotland. This is the first known occurrence of arthropod remains from this locality. Chasmataspids appear to be closely related to eurypterids and fossils of these groups have been confused with each other in the past. Achanarraspis resembles Lower Devonian chasmataspids from Germany, the Midland Valley of Scotland and the Severnya Zemlya Archipeligo, Siberia, but is distinguished from these taxa in possessing a carapace which is distinctly longer than the preabdomen, sharp lateral epimera on all postabdominal tergites, and a distinctive telson morphology. The Middle Devonian age makes Achanarraspis the youngest chasmataspid yet known and significantly extends the known stratigraphical range of the group. Other occurrences of arthropods from the Caithness Flags are reviewed and include conchostracan crustaceans, incomplete eurypterids and a now lost xiphosuran specimen collected in the late 1800s.