The 400 Cheviot sheep, worth around £40,000, were stolen between February and June from the 10,000 acre Littlewood Estate on Donside in Aberdeenshire.
But local sheep farmer Alistair Hay only realised the scale of the thefts after he rounded up his flock last month after the start of the lambing season.
Mr Hay said he had been completely devastated by the loss of his livestock and had been left with only 100 sheep from a 300 strong flock before the ewes began lambing.
He explained that it was likely the sheep had been stolen in a series of raids which began in February after he received call that one of his sheep had been found ten miles away from the field where the sheep had been kept.
Mr Hay, a fencing contractor who works as a sheep farmer part time, said he had been keeping his flock at a remote glen on the Littlewood estate, between Huntly and Alford, since September last year.
He said: “I only realised how many sheep had been taken on 1 June when I brought in the flock. Between the ewes and lambs 400 are missing and I have only got about 100 ewes and their lambs left. There were 300 ewes at the start of the season. It’s absolutely devastating.”
Mr Hay continued: “There have been similar thefts of livestock in the area but nobody, as far as I know, has lost the number of sheep I have. The sheep are kept in a remote glen and nobody would see them up there. I have only had sheep on that hill since September last year and this is the first time this has happened.
“The lambs would only have been about a month old when they were stolen and I think whoever has got them will be holding on to them.”
He added: “Keeping sheep is really a hobby but it’s a huge loss to take. I’ll have to replace the sheep that have been stolen so I will be even more out of pocket.”
A spokeswoman for Grampian Police said the force was investigating the theft of the sheep and urged anyone with information to contact the force.
Last month rural insurers NFU Mutual said livestock rustling had “rocketed” in Scotland last year, with claims rising by 165 per cent. Some farmers are resorting to using CCTV cameras trained on their fields and country lanes to catch the thieves.