The court found the motives raised by the Romanian government as unfounded, the court said in its decision filed on its website on Thursday.
The trade union believes that the IPO and the further privatisation of Hidroelectrica will lead to an uncontrollable loss of government control on the country's natural resources and will compromise the stability of the national energy system, Hidrosind said in a statement on March 18.
The Romanian government gave in January 2012 the green light to the IPO of 10% stakes in hydropower producer Hidroelectrica and nuclear power plant operator Nuclearelectrica in order to raise their share capital.
Romanian property restitution fund Proprietatea holds 19.94% of the company and the remainder is state-owned.
Hidroelectrica was declared insolvent in June, after a long-drawn drought reduced its sales and led to losses.