Foreign Minister Maria Fernanda Espinosa said the 46-year-old Australian, who has been holed up at the embassy for five years to avoid arrest, became an Ecuadoran citizen on December 12.
Assange applied for citizenship in September and was granted it in mid-December 2017.
For more than five years, 2012 to be precise, Assange has has taken refuge at the Ecuadorean embassy in the United Kingdom capital to avoid extradition to Sweden.
Britain said earlier on Thursday it had refused a request by Ecuador for Assange to be given diplomatic status, a spokesman for Britain's Foreign Office said. However, Assange still faces arrest for skipping court bail and could be jailed for between two and 10 weeks if he leaves the embassy.
Moreno issued a warning reminding Assange not to meddle in politics.
More recently, Assange's vocal support for Catalan's independence push drew the ire of Ecuadorian President Lenin Moreno, who was forced to respond to complaints from the Madrid government. U.S. intelligence agencies believe Russian military intelligence used WikiLeaks and other sites to release the emails it hacked from the Democratic National Committee and top Democrats.
The Wikileaks founder has been held up in the Knightsbridge embassy since 2012 following attempt to extradite him to Sweden amid rape and sexual assault allegations.
The announcement came after it was reported that the the British government recently rejected a request that would have granted diplomatic status to and potentially paved the way for him to leave the embassy without risking arrest.
A more public dispute emerged in 2016 when Ecuador cut off Assange's Internet connection due to concerns that WikiLeaks was interfering in the USA presidential election after the organization published hacked emails from the Democratic National Committee.
Assange's USA lawyer Barry Pollack had no comment.
WikiLeaks disclosed emails that led to embarrassing media coverage for Democratic Party candidate Hillary Clinton. Rafael Correa, the president who chose to grant Assange asylum, was widely seen as wanting to burnish his credentials as a leftist leader who would not be cowed by the United States. "Correa used Assange as a propaganda weapon to say that he was a defender of human rights, something that no one in Ecuador believes anymore".
In a separate statement, the ministry said Assange had "committed to not intervening in affairs outside the scope of his asylum status", which seemed an oblique reference to his penchant for creating a stir.
Assange faces both criminal and congressional investigations in the US.