The Prestige oil tanker spilled 50,000 tonnes of oil into the sea when it capsized in the Atlantic in 2002, choking the coasts of Spain, Portugal and France with thick black oil.
Plaintiffs have called for sentences of up 12 years' jail for the Greek captain, Apostolos Mangouras, 78, his Greek chief engineer Nikolaos Argyropoulos and the head of the Spanish merchant navy at the time, Jose Luis Lopez Sors.
After hearing from scores of experts and witnesses, the court in the northwestern city of La Coruña adjourned the trial until the verdict, which officials said was not due for at least two months.
The prosecution has also demanded more than €4 billion ($5.0 billion) in damages overall.
Mangouras blamed the spill on the Spanish authorities which ordered the ship out to sea after it sent out a distress call due to a crack in its hull.
The Bahama-flagged Liberian tanker was carrying 77,000 tonnes of fuel when it sent the call on November 13th, 2002.
Mangouras, along with lawyers representing the shipping company that ran the Prestige, said the order caused it to break up in a storm and spill its load over six days adrift.
"The ship was cracked and they sent it out to the ocean," Mangouras told the court in November. "It was the worst alternative. They sent us in a floating coffin... to drown."
Lopez Sors said he had ordered the ship away from shore to lessen the environmental damage from the spill.
The decision did not spare 1,700 kilometres (1,050 miles) of coastline, which were blanketed with black slime, prompting 300,000 volunteers to come out to clean the beaches.