Nasser Najr al-Mutairi was charged with committing an act of aggression against a foreign nation among others.
On Wednesday the defendants' lawyer, Mubarak al-Shimmari, said: "The criminal court found Nasser Najr al-Mutairi innocent of all the charges against him because of a lack of evidence."
Mutairi, 28, was charged with working for a foreign country and committing an act of aggression against a foreign nation, thus endangering Kuwait's foreign relations, and training in the use of arms.
The public prosecution accused him of fighting against US forces in Afghanistan under the former government, for which he faced a minimum of three years in jail.
Mutairi, the first Kuwaiti to be returned to his country from the Guantanamo Bay prison camp in Cuba, said he went to Afghanistan as a relief worker and does not know how to use arms.
He denied all the charges against him.
Mutairi was repatriated in January under an agreement between Washington and Kuwait.
Eleven other Kuwaitis are still detained at the Guantanamo Bay prison camp in Cuba.