A 12-year-old girl was also injured and was airlifted to hospital in Oban.
Emergency services were called to the scene near the village of Benderloch, north of Oban, at about 14:40 BST on Sunday.
Police Scotland said inquiries were ongoing but the incident was not being treated as suspicious. A report will be sent to the procurator fiscal.
The Health and Safety Executive has been informed about the incident.
The 12-year-old was taken to Lorn and Islands Hospital.
The area where the incident happened is part of the Barcaldine Forest, where there has been logging activity recently.
Margaret Adams, convenor of the local community council, told BBC Radio's Good Morning Scotland programme that the tragedy would have a "massive" impact on the community.
"Even if people don't know the child they will know the family, in a small community," she said.
"It really will have quite an affect on the locals."
Ms Adams said logging had been going on in the area for several months, with signs up warning of the dangers.
She added: "The signs make it very clear that they don't want people to go up because there will be heavy machinery and logs stacked."
Local resident Elaine Walton told BBC Scotland there had been plenty of warnings about forestry operations but it was possible to access the area by avoiding the fenced-off tracks.
"The Forestry (Commission) sent every household in the area a letter telling us the plans for the works, that the place would be sealed off and that there were other walks down at Sutherland's Grove," she said.
"But if you live in the area you know that there are little ways to get up on the hill if you want to and young people explore and find these ways."
A spokesman for Forest Enterprise Scotland said: "Our thoughts are with the family and their friends at this very difficult time and we offer them our deepest condolences.
"We will now focus on working with the site contractor, Tilhill Forestry, and the Health and Safety Executive as investigations into this tragic incident continue."
An eight-year-old girl has died after being trapped by falling logs in a forest in Argyll.