Lasionycta perplexa is a moth of the family Noctuidae. It is widely distributed from southern Alaska and Yukon in the north to California, Utah, and Colorado in the South. A disjunct population is found on the east coast of Hudson Bay at Kuujjuaraapik.
The habitat is conifer forest.
The wingspan is 33–35 mm. Adults are on wing from mid-June through August.
The larvae feed on Alnus species.