Assange received Ecuadorian citizenship in January 2018 as part of a failed attempt by the government of then-President Lenín Moreno to turn him into a diplomat and get him out of its embassy in London.
On Monday, the Pichincha Court for Contentious Administrative Matters revoked this decision.
Ecuador's Foreign Ministry said the court had "acted independently and followed due process in a case that took place during the previous government and that was raised by the same previous government."
Carlos Poveda, Mr Assange's lawyer, said the court made the decision without due process, and Mr Assange was not allowed to appear in the case.
"On the date [Mr Assange] was cited he was deprived of his liberty and with a health crisis inside the deprivation of liberty centre where he was being held," he said.
Mr Poveda said he would file appeals asking for amplification and clarification of the decision.
"More than the importance of nationality,
