The US intelligence agency is accused of recording lawyers' conversations with Assange and copying the data on their mobile devices.
Lawyers for WikiLeaks founder Julian Assange say they are suing the US Central Intelligence Agency (CIA) and its former director Mike Pompeo.
The lawsuit accuses the CIA of recording lawyers' conversations with Assange and copying the data on their phones and computers.
They claim that the agency violated their US constitutional right to privacy with their client.
Assange faces extradition to the US, where he is accused of publishing diplomatic files about the wars in Afghanistan and Iraq in 2010.
The Australian national has appealed the UK's decision to grant the extradition request. The charges against him could result in a prison sentence of up to 175 years.
The complaint against the CIA was filed on Monday by lawyers Margaret Ratner Kunstler and Deborah Hrbek, and journalists Charles Glass and John Goetz.
They claim that the CIA worked with a security firm hired by the Ecuadorian embassy in London, where Assange had taken refuge under political asylum, to spy on the WikiLeaks founder, his lawyers, journalists and others he met.
Lawyer Robert Boyle said that Assange's right to a fair trial had been "tainted, if not nullified" because "the government now knows the content of those exchanges."
"There should be sanctions, up to and including dismissal of these charges or withdrawal of the extradition request, in response to such clearly unconstitutional activities," he told reporters.
The lawsuit also claims that Undercover Global, the Spanish-based security firm, collected data on the lawyers' mobile phones, placed microphones in the Ecuadorian embassy, and sent CCTV images to the CIA.
According to the court document, Pompeo "was aware of and approved" of the operation.