Foreign Minister Ricardo Patino announced the discovery in Quito Wednesday and said he would reveal to whom the hidden microphone belonged to.
The electronic bug was discovered last month in the office of the Ecuadoran ambassador to Britain. Ambassador Ana Alban was at the embassy at the time for a meeting with Assange.
"This is another instance of a loss of ethics at the international level in relations between governments," said Patino, who did not accuse a particular country of planting the device, the London Evening Standard said.
Britain's Foreign Office declined comment on the discovery.