Rwandan soldier-turned-rebel Jean-Paul Bizimana, 31, could face the death penalty when his sentence is handed down later this week for the killings at Uganda's famed Bwindi National Park in March 1999.
Four Britons as well as two Americans, two New Zealanders and a Ugandan game warden, were bludgeoned to death with axes and machetes in the attack.
At the High Court in the Ugandan capital Kampala on Monday, Justice John Bosco Katutsi found that Jean-Paul Bizimana, 31, also known as Xavier Van-Ndame, was part of a group of Hutu rebels who claimed responsibility for the slayings.
"In this case, there can be no doubt that members of the group to which the accused belonged had a common purpose in murdering the tourists," Justice Katutsi said as he announced his verdict. "In this circumstance, each member of the group is guilty of murder.
"I find the accused guilty and convict him accordingly," the judge said.
Bizimana was arrested near the Ugandan border with Rwanda in July 2004 and charged with nine counts of murder, but only went on trial last November.
He denied any participation in the slayings.
Prosecutors maintain that Bizimana and several other suspects still at large led a group of about 100 alleged members of Rwanda's notorious Interahamwe militia in kidnapping the tourists.
The group of 14 tourists was abducted while on a trip to track rare mountain gorillas at the remote rainforest sanctuary made famous in the 1988 film “Gorillas in the Mist”, near Uganda's borders with Congo and Rwanda.
Britons Mark Lindgren, 23, Martin Friend, 24, Steven Roberts, 27, and Joanne Cotton, 28, were among the victims.
Americans Rob Haubner and his wife, Susan Miller, and New Zealanders Michelle Strathern, 26, and Rhonda Avis, 27, also died.
Bizimana’s defence lawyer, Norris Maranga, said he would appeal the guilty verdict.
"Justice has been made at this level but we are not satisfied. Simply being part of the gang does not mean he carried out the killings," Mr Maranga said.
Three other Hutu rebels were arrested in March 2003 and have been sent to the United States to stand trial over the deaths of the two American victims.
The victims were rounded up and forced to march through the jungle during the attack, which devastated Uganda's tourist industry.
After releasing six of the abducted tourists the group used machetes and blunt objects to hack and bludgeon to death the remaining eight, and killed the game warden by setting him ablaze.
Nine people survived, including a French diplomat who was given a note by the rebels warning Britain and the US not to interfere with Rwanda.
Mark Ross, a US survivor of the attack, told reporters after the incident that one of the two young American victims had been raped before she was killed.
Rebel army
Bizimana and his co-accused are all former members of the Liberation Army of Rwanda, which was affiliated to the former Hutu regime in Rwanda.
The organisation is blamed for the genocide of more than a million minority Tutsis and moderate Hutus in 1994.
The Interahamwe played a key role in Rwanda's 1994 genocide in which some 800,000 people, mainly Tutsis and moderate Hutus, were massacred, and have since fought against the current Rwandan government.
Before joining the Interahamwe, prosecutors said Bizimana had been a Rwandan army soldier.
In a message to authorities given to the French envoy, Anne Peltier, the group said they had killed the tourists because of US and British support for Rwandan President Paul Kagame, who led the rebel group that ousted the Hutu-led regime blamed for the genocide.
"The Interahamwe are not happy with the American and British because they have preferred to support the Tutsi ethnic minority against the ethnic Hutu majority," the message said.