The advocacy group said it obtained documents from Libya that indicate the country's oil revenues were grossly mismanaged under Moammar Gadhafi's government.
Global Witness said the documents suggest low-quality crude oil was sold on false pretenses to Exxon Mobil, which cost the company about $4 million. Other companies like Norwegian fertilizer company Yara received "large discounts" on natural gas prices from the Libyan National Oil Co.
"Murky dealings within Libya's National Oil Company, and the systematic mismanagement of the country's oil wealth have effectively denied millions of dollars to the people of Libya," said Giulio Carini, a campaigner at Global Witness.
Italian energy company Eni revealed in early April that it was being investigated by the U.S. Securities and Exchange Commission for alleged illegal payments to Libyan officials.
Global Witness called on the Libyan interim government to release all of its records on oil contracts for the sake of transparency.
"The case for reform of the country's oil sector could not be stronger or more urgent," said Carini.