The men were also cleared of fighting US forces in Afghanistan, judicial sources said on Sunday.
The US released the men in November, but on their return to Kuwait they were re-arrested and charged.
The men's defence lawyers argued that the trials were politically motivated and that the men's reported confessions were obtained by US interrogators at the Guantanamo facility.
The prosecution reportedly plans to appeal the ruling.
About 460 detainees are being held in Guantanamo Bay in Cuba, including six more Kuwaiti nationals. All are being held without charges against them or trial.
Rights groups in Kuwait welcomed the decision to free the men.
"This ruling underscores what we were sure of all along, that our sons are innocent"
Khaled al-Odah, detainee rights activist
"This ruling underscores what we were sure of all along, that our sons are innocent," Khaled al-Odah, head of a detainee support committee.
"They have been imprisoned unjustly for years at the American base and [this is] a stark breach of international and humanitarian laws by the American administration," he added.
Kuwait is a staunch US ally and host to thousands of US troops. It was the launching pad for the US-led invasion of Iraq in 2003.
Last week the US also released 15 Saudi men from the Guantanamo prison. They arrived back in Saudi Arabia on Friday.