Astaena marginithorax is a species of beetle of the family Scarabaeidae. It is found in Ecuador.
Adults reach a length of about 11 mm. They are blackish-brown and strongly glossy. The elytra are punctate in rows, with the intervals irregularly and very sparsely covered with punctures. The underside is light reddish-brown, with the legs slightly darkened. The antennae are brown.