Halet in 2012 had forwarded a series of documents from his employer, PwC, to journalist Edouard Perrin, who used them in a June 2013 broadcast about the LuxLeaks revelations.
The documents were later posted online by the International Consortium of Investigative Journalists which helped break the story of tax avoidance by multinationals in the grand duchy.
PwC was implicated in the revelations as a negotiator of lucrative tax rulings between companies and the government.
“The Court held that the domestic courts had struck a fair balance in the present case between the need to protect the rights of the applicant’s employer on the one hand and the need to protect Mr Halet’s freedom of expression on the other,” the European Court of Human Rights said in a statement.
A Luxembourg court of appeal had fined Halet €1,000 for his role in the revelations as well as a symbolic sum of €1 in compensation to PwC, saying that documents were subject to professional secrecy and their revelation had caused his employer harm.
Halet’s appeal against the decision was dismissed in January 2018, leading him to lodge an application with the European Court of Human Rights in May 2018.
The ECHR called the penalty “mild”, saying it would have “no real chilling effect on the exercise of the applicant’s freedom or that of other employees.”
The 16 documents forwarded to Perrin by Halet pale in comparison to the cache of 45,000 confidential documents previously forwarded to the journalist by whistleblower Antoine Deltour.