The 20 carat golden nugget, which weighed about 18.1g (0.6oz), has an estimated value of £10,000.
It was discovered by a Canadian man during a gold panning course near Wanlockhead in the Lowther Hills.
However the man, known as John, was so unimpressed by his discovery, he almost threw it back in the water.
He said: "We were a few minutes into panning when I found the nugget with a bean tin, an underwater viewing tin with a glass base.
"I saw the piece lying just under the surface of the water and picked it up. It didn't look like much and I was just about to throw it back when I thought I should double-check with Leon first.
"I said 'is this gold?' and couldn't understand his excitement.
"Looking at the size of the piece I still can't believe it's such a big deal but it's very interesting to watch the reactions."
His instructor, Leon Kirk, said the last significant find of Scottish gold was a fraction of the size.
It was found in the area in 2002 and it weighed 4.1g (0.14oz).
"I like to tell my students that everyone goes home with a flake or two of gold, but this is exceptional," Mr Kirk said.
"I always knew there were sizable nuggets in these hills but, in my decades of panning, have only found very small pieces or flakes."
He added: "This is such a major discovery that people will be talking about it in 30 years' time."
Gold nuggets of this size are extremely rare in Scotland and any that are found command a premium price.
Mr Kirk said: "I am over the moon. This find leaves a really positive message that there are still big nuggets out there to be found just a few inches beneath the water."
The gold panning course operates out of Wanlockhead's Museum of Lead Mining.
Museum trustee Gerard Godfrey said: "We are absolutely delighted with such a significant find. No doubt there will be a mini gold rush once this gets out but we are very happy with that.
"We welcome responsible panners to the area and have all the equipment and information necessary for experienced or novice enthusiasts.
"There is now no doubt that there are some very large pieces of gold in these hills which are just waiting to be discovered."
A nugget of gold found in a river in the Southern Uplands is thought to be the most significant discovery in Scotland in the past 70 years.