Greenpeace said it bought foreign currency at a fixed exchange rate at a time when the value of the euro was growing. It lost $5.2 million in the bet the euro would decline in value against other currencies.
"We offer a full apology to our supporters for the series of errors that led to the loss," the group said in a statement Sunday.
Greenpeace attributed the loss to a "serious error of judgment" by an employee who acted without proper authority. The advocacy group said the employee, who has since been fired, made no personal gain from the trade.
"We further wish to reassure people that every possible action is being taken to avoid the possibility of such a loss ever occurring again in future," it said.
For 2013, the advocacy group said it had a budget deficit of $9.2 million.