NFUS made the discovery at a store at Hermiston Gait near Edinburgh.
The union said it discovered shelves full of imported New Zealand lamb, all sitting under a banner which claimed it to be ‘The Best Scottish Lamb in Season’.
NFUS contacted the UK’s biggest retailer on Friday September 11 asking it to address the situation.
NFUS asked Tesco to check point of sale information is accurate at all its Scottish stores and to live up to its pledges by immediately increasing its commitment to buying home-produced lamb.
To add to the ire of Scottish sheep farmers, the discovery comes at peak season for Scottish lamb.
The annual ‘Wham Bam Lamb’ campaign promoting Scotch Lamb has been running in recent weeks.
This has included TV and radio advertising and the ‘Love Scotch Lamb’ weekend, which saw lamb tasting activities at various venues - including Tesco stores.
With the difficult year for lamb prices added, the discovery is 'unacceptable', according to NFU Scotland president Allan Bowie.
He said: “The deep felt anger being directed towards Tesco over this debacle is wholly justified. This is totally unacceptable. Scotland’s sheep farmers and our consumers deserve much, much better than this.
“First and foremost, why is Tesco still promoting imported lamb in shelf-filling quantities when this is peak season for Scottish and UK supplies? This makes its well-publicised pledges of support for Scottish and UK farmers appear wafer thin.
“Secondly, this kind of point of sale abuse of the ‘Scottish’ brand makes Scottish farmers’ blood boil.
"It is difficult to put into words just how angry hard-working sheep farmers feel when they spot some excellent promotion of Scottish lamb in a Tesco store only to find the shelves below are filled with imported product."
He said consumers looking to support Scottish farmers would often not realise the lamb they were buying 'does nothing to support our sheep sector here in Scotland'.
“By way of a response, we will not be fobbed off by Tesco telling us that this is a one-off or that it was simply human error because we have had that excuse before." he added.
“The UK’s biggest retailer must do better. It must urgently review all of its signage in Scottish stores and ensure that where it proudly proclaims to be offering ‘The Best Scottish Lamb in Season’, then that is what is on the shelf."
“There are other major retailers who can successfully stock Scottish lamb 12 months of the year. A far greater commitment from the UK’s biggest retailer to sourcing home-produced lamb would make a massive difference.
"That is something that we have been discussing with the retailer for some considerable time but it is time for Tesco to live up to its pledges or they will be viewed as little more than lip service."
Tesco insisted it remained a strong supporter of Scottish lamb, pointing out it did not imply the store only sold Scotch lamb.
A spokesperson for Tesco said: “We recognise the quality of Scotch lamb and we are proud to be its biggest customer, and we sell more Scotch lamb than any other retailer.
“All of our lamb counters are 100 per cent Scotch lamb all year round and our Tesco Finest lamb is also Scotch. Our pre packed lamb, which currently is predominantly Scotch, is clearly labelled with the country of origin to allow our customers to simply identify Scotch products as part of their shopping choices.
“As a long-standing champion of Scottish agriculture, we are supporting Quality Meat Scotland with their ‘Wham Bam Thank You Lamb’ tour, which is visiting 30 Tesco stores across country, to showcase and offer customers the opportunity to sample Scotch lamb.
"Furthermore, another 15 Scottish Tesco stores are showcasing Scotch lamb as part of our ‘Love Local’ campaign."