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.

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