In a graphene-semiconductor heterojunction photodetector and a method of manufacturing the same according to the present inventive concept, a source electrode and a test electrode are formed to face each other on a graphene layer, and a drain electrode is formed in a direction perpendicular to a central region portion of the graphene layer, so that the drain electrode may be physically separated from the graphene layer. Further, charges formed at the central region portion of the graphene layer are transmitted to the drain electrode through a substrate, so that high photosensitivity may be secured, and a high output voltage may be secured for the applied light. Accordingly, the drain electrode is formed at a side surface of the graphene layer, so that the size of the drain electrode may be easily controlled, and a high output voltage may be obtained.