Initial speculation about the shooting had focused on Iran, which denied the accusations. The suspects told investigators they were trying to rob the diplomat and shot him accidentally.
Khalaf bin Mohammed Salem al-Ali, a 45-year-old official in the Saudi Embassy's consular section, was killed near his home in Dhaka in March.
Tribunal Judge Mohammad Motahar Hossain handed down the verdict Sunday, chief prosecutor Rafiqul Islam said. One of the men was tried in his absence, Islam said.
He said the men can appeal the verdict.
Iran has been accused of other international attacks or attempted attacks against diplomats, including Saudis. Days after the shooting, Saudi Arabia sent investigators to assist Bangladeshi detectives.
The defendants pleaded not guilty at the trial. After their arrest in July, the four men told investigators they tried to rob the diplomat as he was going for a walk on the deserted street and shot him accidentally during a scuffle.
Police said the men were arrested after a revolver and a car used in the killing were found in their possession.
Muslim-majority Bangladesh enjoys good relations with Saudi Arabia, which is a top destination for Bangladeshi migrant workers.
Relations between the countries were tested in October last year, when Saudi Arabia beheaded eight Bangladeshi workers who were found guilty of robbing and killing an Egyptian.