The incident happened on grassland in Badagyle, near Ullapool.
A tourist spotted the distressed animal, raised the alarm and the farmer managed to free the can from its mouth.
Noel Hawkins, from the Scottish Wildlife Trust, said: "The can was lodged on its lips, it couldn't eat or drink.
"If the can hadn't been removed, it would have died. It would have been a long slow death."
He said visitor numbers to the Scottish Highlands had greatly increased owing to the North Coast 500 tourist route, which was launched in 2015.
"It's a scenic part of the world but the North Coast 500 has brought a lot of people in, and some of those passing are throwing rubbish out their cars," he added.
"The farmer was upset - I'd seen him a few days before and we'd talked about the litter.
"Rubbish isn't just on beaches, it's everywhere. It's quite horrible. I've seen birds getting things jammed on, but not livestock. Sheep will eat anything."
On this occasion the sheep was unharmed, but Mr Hawkins urged people to be aware of the risks posed by discarded rubbish.
He said: "Often animals are left incapable of eating and drinking and can die a slow and agonising death.
"We are asking people to bin and dispose of their rubbish properly and to help pick up things that others throw away.
"Not only does rubbish make our countryside look bad but is a danger too."