The 16-year-old was attacked next to the pier in Balloch Country Park at around 5.40pm on Tuesday.
He was taken to the Royal Alexandra Hospital in Paisley and treated for serious arm and head injuries.
Loch Lomond assault: A teenager has been seriously injured in an assault near the Loch Lomond pier
Police are appealing for anyone who witnessed the assault to come forward.
Anyone with information can contact police by phoning 101.
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