Besigye has disputed President Yoweri Museveni's victory in February's election and has been under house arrest for much of the time since.
He was arrested in the capital Kampala on Wednesday after staging his own swearing-in ceremony and whisked to Moroto, a town in the remote Karamoja region of the country.
"We have been informed by police today that Besigye appeared in court in Moroto and was charged with treason," lawyer Erias Lukwago told AFP.
"It's absurd because none of us was there to represent him," he said.
Treason is a capital offence in Uganda but the death penalty has not been carried out for years. Besigye was previously charged with treason in 2005 but the case against him was eventually dropped
Police spokesman Fred Enanga confirmed the latest case against Besigye but was unable to provide any further details of the charges.
A long-standing opponent of Museveni, Besigye has been frequently jailed, placed under house arrest, accused of both treason and rape, tear-gassed, beaten and hospitalised over the years.
