Roy Bennett, a senior member of Zimbabwe's Movement for Democratic Change, has been found not guilty of trying to overthrow Robert Mugabe, the president.
Bennett was arrested last year on charges of possessing weapons with an intent to commit sabotage, terrorism, banditry and insurgency.
But Zimbabwe's high court ruled on Monday that there was not enough evidence to sustain the charges.
"The state has failed to prove its case," the judge told the court as he dismissed the charges.
"The accused is accordingly found not guilty."
Bennett is a close aide to Morgan Tsvangirai, the prime mininster in Zimbabwe's power-sharing government. The MDC shares power with Mugabe's Zanu-PF following a disputed election in 2008 - but frequent policy disputes mean there has been little unity within the unity government.
Little agreement over ministries
The last two years have been marked by frequent disagreement over the appointment of government ministers, particularly the attorney general and the governor of the central bank.
Tsvangirai insisted that the charges against Bennett were fabricated to keep him out of the government; Bennett had been poised to become the deputy minister of agriculture before the accusations.
"I am very happy. This judgement gives hope that we are returning to justice and the rule of law," Bennett told reporters after the ruling.
The case dates back to 2006, when a weapons dealer was arrested and initially accused of plotting to assassinate Mugabe.
The dealer has said that police tortured and forced him to make a false confession implicating several people from both Mugabe's party and Tsvangirai's party, including Bennett, in the alleged plot.
The dealer, who was eventually was cleared of treason but charged with possessing illegal weapons, was a key witness in Bennett's trial.