Transocean Deepwater Inc has been convicted of breaching clean water laws for its part in the Deepwater Horizon oil rig disaster in the Gulf of Mexico in April 2010.
The firm, which owned the drilling platform that exploded, killing 11 rig workers and leaking five million barrels of oil from BP's Macondo well into the ocean, was ordered by a US federal judge to pay $400m to settle criminal charges, reported NOLA.com.
The company will also serve a five-year probation period after pleading guilty to breaking the Clean Water Act. Transocean has already agreed to a separate $1.4bn in civil and criminal penalties with the Department of Justice.
The fine follows a $4.5bn settlement by BP, the largest in US history, for its role in the disaster, which devastated the environment and bird, fish and animal life. It took three months to plug the leaking well.
Within the fine were $1.25bn in criminal penalties, $2.4bn to the National Fish and Wildlife Foundation, a $525m civil penalty to the Securities and Exchange Commission and a $350m payment to the National Academy of Science.
BP had already agreed in 2012 to pay $7.8bn to cover the economic, property and pollution-related damages linked to what has been called the worst environmental disaster in US history. It may face a further $21bn payout in another civil case over claims that it broke the same clean water laws as Transocean.