The United States Department of Agriculture (USDA) recommends cooking all fish – including tuna – to an internal temperature of 145 degrees. At this temperature, tuna is considered fully cooked and safe to eat. However, heating tuna to such a high temperature can lead to overcooking if you aren't careful. To maintain the tender texture that most people appreciate from tuna steaks, chefs often recommend serving tuna steaks medium-rare, which is about 125 degrees. It isn't easy to estimate the internal temperature of fish such as tuna steaks, and that isn't something you'll want to guess about anyway. Have a meat thermometer handy to check that the internal temperature is safe before serving.