"The time has come for true unity and our priority today is for the opposition to sit down with the protesters at Pearl Square and clearly set our demands," Mashaima told reporters at his home.
Mashaima was charged in October, along with 25 men, with forming an illegal organisation, financing terrorism and spreading misleading information.
He was in Britain for medical treatment when the charges were brought against him, and he stayed there until his group was granted a royal pardon from King Hamad this week.
Mashaima, a Shiite-Muslim, says he will join protesters - also mostly Shiite - who have been camped out in Manama’s Pearl Square, calling for the Sunni al-Khalifa dynasty to be replaced by an elected government.
Official opposition groups have not been demanding regime change, but instead they have called for major reforms, including an elected prime minister and the creation of a real constitutional monarchy.
Opposition MPs, 18 of whom withdrew from parliament last week to protest the killing of seven protesters, said Saturday they were waiting details on a proposed dialogue with the government before they agree to talks.
King Hamad has charged Crown Prince Salman with opening dialogue with all parties in Bahrain, but those talks have not yet started.