Tuviras (Gymnotus sp.) are a small fish widely used as live bait in sport fishing. Interest in its culture has increased, but there are still several bottlenecks that need to be solved. For example, the process of sexing in tuviras is difficult since they do not present sexual dimorphism. Thus, the aim of this study was to verify if an abdominal puncture could be used as an inexpensive, fast and reliable technique to identify the sex of adult Gymnotus sp. Before performing the abdominal puncture, five individuals were dissected to determine the exact location of the gonads. Based on this information, the abdominal puncture was performed 6 cm away from the tip of the animal's head and 1 cm below the lateral line with an intravenous infusion set connected to a 5 ml syringe. An abdominal puncture was performed on 24 adult animals that were larger than 20 cm in length. Twenty‐two individuals were sexed correctly, and the accuracy degree was 91.66%. The survival rate of the animals 96 hr after the abdominal puncture was 100%. These results indicate that the abdominal puncture is an easy, simple, fast and inexpensive technique that can be performed in a reliable way.