These insects look like large, slender wasps. They have a yellow head with a dark stripe between the eyes. The thorax and abdomen are dark, either black or dark brown. Their long legs are mainly yellow, while their four elongated, narrow wings are almost black.
Females have an impressive 12- to 15-cm long dark brown ovipositor.
This species resembles two other ichneumons found in the same habitat in Quebec: Megarhyssa macrurus and M. greenei. Giant ichneumons are recognizable by their blackish-brown abdomens. In addition, they have the longest ovipositors.