During the attack, in April in the town of Mbandaka, local militia overwhelmed UN and government troops and briefly seized the airport.
The unrest followed clashes between two ethnic groups over fishing rights.
Some 36 men were initially charged; four were acquitted and the rest were given prison terms.
Defence lawyer Steve Omekongo told the BBC he would appeal against the death sentences.
The unrest in Mbandaka, the capital of Equateur province, is separate from the violence involving numerous armed groups in eastern DR Congo, which has drawn the world’s biggest peacekeeping operation to the country.
Mbandaka airport was retaken the day after the attack in a joint operation by DR Congo government troops and UN peacekeepers.
A rights group claimed that Congolese troops had killed civilians during the operation.
A government spokesman told the BBC it was the first time army killings in Mbandaka had been reported.
Mbandaka is the capital of Equateur province, where at least 100 people were killed in clashes between the Lobala and Boba communities in 2009, displacing an estimated 200,000 people.
Source: BBC