The study, by Oceana environmental group, says more than a third of fish is being mislabelled in and around Brussels' European Union institutions.
In EU canteens, where many European officials eat, 38% of fish is wrongly labelled.
Oceana's executive director, Lasse Gustavsson, called it "total chaos".
"The EU needs to clean up its fishy business, take responsibility and urgently improve traceability and labelling of seafood," he said.
The investigation inspected DNA from over 280 samples in 150 restaurants in Brussels' tourist and EU areas. Overall, 31.8% of fish was mislabelled.
Oceana says that in 14% of cases, cod was replaced by the less expensive pangasius, a type of catfish.
Around 95% of what was labelled bluefin tuna - a sushi favourite - was in fact the cheaper yellowfin alternative.
Sole was substituted 11% of the time with other flatfish.
The report is the latest in a string of "food fraud" cases. A recent study for Which? found a quarter of dried oregano sold in shops in the UK and Ireland contains other ingredients.
In a more drastic case, a criminal ring in China was found selling rat, fox and mink meat as mutton.
Diners in restaurants in the Belgian capital may be in for a surprise after a report found 30% of fish is being substituted with cheaper alternatives.