A new species of Diadocidia Ruthe fungus gnat was discovered in a mixed pine and hardwood forest in the eastern Lower Peninsula of Michigan. Diadocidia longivena Taber is the fourth species recorded from North America and the first new species of the genus described from the Nearctic in 63 years. The fly belongs to subgenus Adidocidia Laŝtovka and Matile, as do both of its congeners that were described in North America.