The ministry of the environment is arguing that the verdict, delivered last week, is too lenient, ThinkSpain reported.
The ministry is contesting the acquittal of the American Shipping Bureau, which found the Prestige oil tanker seaworthy before the spill off the coast of Spain in 2002.
As part of the verdict, Captain Apostolos Mangouras was sentenced to nine months in prison for disobeying authorities, but was acquitted of an environmental crime.
He faced up to 12 years in prison.
Another ship officer, Argyropoulos Nikolaos, and Jose Luis Lopez-Sors, former director general of Spain's Merchant Marine Department, were acquitted of all the charges against them.
The ship's hull was gashed during a storm in November 2002 and some of its 70,000 tons of fuel oil gushed out, covering hundreds of miles of Spanish coastline.
Days later, the ship sank and oil continued to leak from the vessel as it sat at the bottom of the Atlantic.